Some opinions rule that before Purim one must give half of his country’s coin currency [to charity, and so is the custom of all Jewry]. It is given as a remembrance for the half shekel that was donated for purchasing sacrifices during the times of the Temple in the month of Adar.
To what purpose is the half shekel to be donated to? Some Poskim rule the money is to be given specifically to paupers and not to a synagogue and the like. Others however rule one may use the money for a Shul and other Mitzvah purposes. The Rebbe writes that one is to especially give the donation for the purposes of a Shul just like the original Machatzis Hashekel was used for the Beis Hamikdash. This however does not diminish from the option to give the money to actual paupers.
Who is obligated to give the half Shekel donation and must one give on behalf of his family? Some Poskim rule that only [men] over the age of twenty are obligated to give the half Shekel, while those under twenty are exempt. Others however rule that all boys over the age of Bar Mitzvah are obligated. It is likewise disputed amongst Poskim whether women are obligated to give the half shekel. Practically the custom is for the father of the house to give three half coins on behalf of each member of his family, including his [wife and] small children [both male and female], including even on behalf of the fetus of a pregnant woman. [Furthermore, it is best to give the children the money for them to fulfill the Mitzvah themselves.]
If a father gave on behalf of his children one year must he continue giving every year on their behalf? Once a father has begun giving a half Shekel on behalf of his [male] children he must give on behalf of his [male] children every single year [until they reach the age of twenty].
When is the donation to be given? The half Shekel donation is given on Erev Purim prior to Mincha. [Some however have the custom to give it after Mincha, and so is the widespread custom in Jerusalem. In Jerusalem, and other walled cities, it is to be given on the day of the 14th, prior to Mincha, and not on Taanis Esther.] Some have the custom to give it prior to Megillah reading of the day. [Due to this difference of custom, some are accustomed to give the Half Shekel donation twice, once on Erev Purim before Mincha, and once on Purim prior to the Megillah reading.]
When Purim falls on Sunday when is the half shekel given? When Purim falls on Sunday the half Shekel is given on Taanis Esther, which is on the previous Thursday. [However in Jerusalem and other walled cities it is given on Sunday.]
How much is one to give? One is to give three coins to charity. Each coin is to be half of the currency amount used in his country. Hence in the Unites States that the currency is one dollar one is to give three half dollar coins. In Israel that the currency is one Shekel one is to give three half shekel coins and so on and so forth for the currency of each country. [Some Poskim however rule that if one has the ability, he is to give a half coin which contains real silver and is worth at least the monetary value of the half Shekel in the times of the Temple. If such a coin is not available then he should give other coins which add up to the monetary equivalent of the half Shekel, as it was worth in the times of the Temple. The half shekel was originally worth 9.6 grams of pure silver. The value of the half Shekel thus fluctuates in accordance to the price of silver. [As of January 2015 one gram of silver was worth $.57 for a total of $5.47 for the half silver Shekel. Each Purim one must verify the current market price for silver, according to this opinion.] Thus one who desires to fulfill the Mitzvah in its highest form is to give this amount of money to charity. If he cannot afford it, he may give the half of the country’s currency coin as stated above.]
Exchanging the half coin with the Gabaiy: It is a common custom in some communities throughout the world, including Eretz Yisrael, for the Gabaiy of the Shul to prepare three half dollar coins and each person exchanges it for their value and places the three half dollar coins in charity.
The half Shekel donation is given on Erev Purim prior to Mincha. When Purim falls on Sunday the half Shekel is given on Taanis Esther, which is on the previous Thursday. However in Jerusalem and other walled cities it is given on Sunday. One is to give three half coins to charity. Each coin is to be half of the currency amount used in his country. The custom is for the father of the house to give three half coins on behalf of each member of his family including his wife and small children [both male and female], including even for a fetus of a pregnancy. It is best to give the children the money for them to fulfill the Mitzvah themselves.
Must one specifically give half dollars as Machatzis Hashekel?
In the U.S.A one is to give half dollar coins, as that is half of the American currency. Other countries are to give half of their currency coin. Many however are accustomed that even outside of the U.S. the Gabaiy of the Shul prepares three half dollar coins which is then exchanged. See the above Halacha and footnotes there for an elaboration on this topic.
May one switch half coins from a Pushka?
The custom is to do so although some question this custom.
Is one to donate all three half coins simultaneously or one after the other?
There is a difference in custom in this matter. The widespread custom is to give all three coins simultaneously for each family member.
Is one to say “Zecher Lemachatzis Hashekel” upon giving the donation?
Some have the custom to say “Machatzis Hashekel” upon giving the donation. However some Poskim rule that one may not say this and must rather say “Zecher Lemachatzis Hashekel” upon donating the coins. Others rule it is forbidden to say anything at all, and one is to give it without reciting its purpose. Practically many have the custom to say “Zecher Lemachatzis Hashekel”.
What is one to do if he does not have any half coins available?
He is to donate three whole coins with intent that half of each coin is being given as Machatzis Hashekel and the other half as a present.
May one use Maaser money for Machatzis Hashekel?
Must a pauper who lives off charity give Machatzis Hashekel?
This matter requires further analysis.
The meaning of the half Shekel and its connection to Purim:
The Gemara states that Haman gave Shekalim on behalf of destroying the Jewish people and in order to counteract his Shekalim, Hashem gave us a Mitzvah of giving Shekalim. The Mefarshim explain that Haman wanted the moneys to be distributed to charity in order to gain Divine consent for the successfulness of his plot. This is why the Gemara states that the Jews already preceded their Shekalim to the Shekalim of Haman, as they preceded their charity to Haman’s charity.
 Rama 694/1
 The Rama writes that only some opinions rule one is to give a half Shekel before Purim, as the Rambam, Tur and Michaber made no mention of this opinion, hence implying they do not hold of it. Nevertheless the custom even amongst Sefaradim is to give the half Shekel before Purim as writes the Rama. [Kaf Hachaim 694/19]
 Kaf Hachaim 694/19
 Kaf Hachaim 694/19
 Shaareiy Teshuvah 694/2; Shaar Hatziyon 694/9
 Minchas Elazar 1/30; Kaf Hachaim 694/22
 Sichas Parshas Shekalim 1964 and 1982; Sefer Hasichos 1990 Vol. 1 p. 303, Likkutei Sichos Vol. 1 p. 182; Sefer Hasichos 1991 Vol. 1 p. 320
 M”B 694/5
 Rama ibid; Rav Ovadia Bartenura
 Tosafus Yom Tov brought in M”A 694/3; M”B 694/5
 Hagahos Maimanis writes that women are obligated to give the Half Shekel. The M”A 694/3 however questions this ruling saying he does not know of a source for it. Kaf Hachaim 694/27 explains the source for the Hagahos Maimanis is from the words “Lechaper Al Nafshoseichem” and since women also need atonement therefore they are also to give.
 Daas Torah 694; Orchos Chaim; Leket Yosher (student of Terumos Hadeshen); See Darkei Chaim Veshalom 843; Sheivet Halevy 7/183; Piskeiy Teshuvos 694/5 footnote 31; Sefer Haminhagim p. 172 [English Edition]
Background and Other Opinions: The M”B 694/5 omitted the custom of giving on behalf of one’s wife and female children [although he does write a pregnant woman gives on behalf of her fetus], as rules the M”A 694/3 that women are exempt from giving the half Shekel. [Piskeiy Teshuvos 694/5] So also rules Siddur Yaavetz that one only gives on behalf of his male family members. Nevertheless despite these opinions the custom is to give the half Shekel on behalf of all family members including one’s wife and female children. [Daas Torah 694; Orchos Chaim; Leket Yosher (student of Terumos Hadeshen); See Darkei Chaim Veshalom 843; Sheivet Halevy 7/183; Piskeiy Teshuvos 694/5 footnote 31.]
The Chabad custom: In Sefer Haminhagim p. 172 [English Edition] it is recorded the custom of the Rebbe to give the half Shekel on behalf of his family, including his wife and children. Although there it is stated explicitly that this is not meant as a directive for the public, nevertheless in Sichas Taanis Esther 1989 the Rebbe explicitly stated that the custom is to give for one’s entire household.
 M”B 694/5; Poskim brought in previous footnote; Sefer Haminhagim p. 172 [English Edition]
 See previous footnote and Poskim there
 Darkei Moshe 694/1 “A pregnant woman gives on behalf of her fetus”; Elya Raba brought in M”B 694/5; Kaf Hachaim 694/24
The reason: The above custom to give three half coins even on behalf of a fetus is hinted to from the verse “Zeh Yitnu Kol Haubar” which through a play of words can be translated as fetus. [Darkei Moshe ibid]
 Sefer Hasichos 1992 p. 787
 M”A 694/3
 However regarding female children, or one’s wife, one does not have to give on their behalf every year if he does not choose to do so. [Tiferes Adam 3/21; Piskeiy Teshuvos 694/5]
 The reason: As according to the Rama until the age of twenty the children are exempt. However according to those which obligate the child from the age of 13 then from that age and onwards the father is exempt on giving on his behalf.
 Rama ibid in name of Mahril. The Rama ibid concludes “So is the custom in all of these provinces”.
 Luach of Rav Y.M. Tukichinsky
 Luach Kolel Chabad; Vintage and widespread custom in Jerusalem
The Rama [694/1] rules the half Shekel is to be given prior to Mincha on Erev Purim, thus implying that it does not contain a connection to Taanis Esther. Thus in walled cities, such as Jerusalem, that Purim is celebrated on the 15th, it is to be given on the 14th before Mincha, and not on Taanis Esther. This follows the instruction in Luach Kolel Chabad and so is the vintage and widespread custom in Jerusalem.
Other opinions: The Kaf Hachaim [694/25 based on Darkei Moshe, and Siddur Rashash] rules that the half Shekel is to be given specifically on Taanis Esther and hence when Purim falls on Sunday one is to give the half Shekel on Thursday, which is Taanis Esther. The reason for this is in order to connect the giving of the half Shekel to the fast day which is a day of atonement. [ibid] Likewise, in Sefer Haminhagim p. 170 [English Edition] it states that the half Shekel is given on Taanis Esther even when Purim falls on Sunday, hence implying the connection with a fast day.
 M”A 694/2 [brought in M”B 694/4] writes that the custom in his country is to give the donation on Purim prior to reading the Megillah in Shacharis.
 Darkei Chaim Veshalom 843; Divreiy Yatziv 2/292
 Sefer Haminhagim p. p. 170 [English Edition]; Kaf Hachaim 694/25; Darkei Chaim Veshalom 843
 Luach Kolel Chabad; Vintage and widespread custom in Jerusalem; See Background and other opinions stated above!
 Rama ibid
The reason: This is in commemoration of the three times that it states the word Teruma in the Parsha. [Rama ibid] Alternatively it corresponds to the three days of fasting instituted by Esther. [Hisvadyos 1989 2/439]
Custom of four coins: The Rama writes that some opinions say one is to give 4 half coins to charity, meaning another half coin in addition to the three half coins. Practically he concludes that the custom is not like this opinion.
Other Opinions: Some Poskim question the custom to give three half coins to charity in correspondence to the three times is says Teruma, as in truth there is only one time that the word Teruma refers to the half Shekel donation, while the other two Terumas refer to the one time donation given to the Mishkan at the time of Moses. Hence why the need to commemorate these other two Terumas if they were never repeated even in the times of the Temple. They thus conclude that in truth one is only required to give a single half Shekel, and so is the custom in certain places. [Mateh Yehuda 694; Maaseh Rav of Gr”a 233 brought in Kaf Hachaim 694/23]
 So rules Rama ibid and so is the custom.
Other Opinions: Some rule there is no need to give a half coin to Tzedakah and rather any amount suffices in commemoration of the charity. [Minchas Elazar 1/30; Darkei Chaim Veshalom 843] It may even be given as a bill or check. [Piskeiy Teshuvos 694/4]
 Se however next part in this Halacha that even in Israel some have a custom to give three half dollars.
 Kaf Hachaim 694/20; Shiureiy Torah p. 294-295; See Rambam Shekalim 5 that the half coin used for the half Shekel Mitzvah was never worth less than the half shekel in the times of Moshe.
 The half Shekel in times of the temple contained 9.6 grams of pure silver. [Piskeiy Teshuvos 496/4]
 Shiureiy Torah p. 294-295; A silver Shekel is 6 Darham of silver [19.23 grams]. Thus a half a shekel is 3 Darham [9.615 grams]
 One can visit  Luach Tukichinsky; See Biur Halacha “Veyeish Litein” which questions the ability for one to exchange the three half dollars from charity once they have already been donated by the first person. Nevertheless, as he concludes there, this is the custom of many areas.
 The reason: The reason for this is because the half Shekel coin was not around 60 years ago and hence they used the half dollar. This custom has continued until today in certain areas. Another advantage of using the half dollar is because it contains real silver, and as rules the Kaf Hachaim, there is advantage in giving the half Shekel with a coin that contains real silver. Nevertheless this only applies to half dollar coins which were minted prior to 1971. In 1971 the U.S. ceased to mint coins with silver, and hence the half dollar coins minted from that year and onwards do not contain any silver. Hence it is only advantageous to use the half dollar in places outside of the U.S. if it was minted before 1971. [See Piskeiy Teshuvos 694 footnote 25] The Kennedy half dollar coins however still contain real silver even for those minted past 1971 until the present.
 In Eretz Yisrael the custom today of those which give a half dollar coin, is to exchange the three half dollar coins for three half Shekalim, which is below their value. The Piskeiy Teshuvos [694 footnote 25] explains that this is done in order to suspect for the opinion of the M”B which invalidates exchanging the money from charity, and hence they give three half Shekalim in order to suffice also for his opinion. Vetzaruch Iyun as once the coins have been placed in charity how can one exchange them for less than their value?
 See Piskeiy Teshuvos 694/4
 Biur Halacha “Veyeish Litein”. He however concludes there that since this is the custom perhaps it is considered as if one stipulated with the charity that the coins may be exchanged.
 Piskeiy Teshuvos 694 footnote 26
 Piskeiy Teshuvos 694 footnote 27
 Zecher Simcha 76 defends this practice
 Shut Hageonim 40 [I have not found the source written there or anywhere in Shut Hageonim, however so writes Piskeiy Teshuvos ibid]
 As if one says they are Machatzis Hashekel the coins become Hekdesh and may not be used for any purpose and must be buried. [ibid]
 Divrei Yosef 52
 Piskeiy Teshuvos ibid; Yalkut Yosef p. 311
 Daas Torah 694; Tzitz Eliezer 13/72; Minchas Elazar 1/30 rules there is no need to give a half coin to Tzedakah and rather any amount suffices in commemoration of the charity.
 Shlah 260b; Elya Raba 686/4; Kaf Hachaim 694/14
 P”M brought in Biur Halacha 694 “Yeish Omrim”
 Megillah 13b
 Iyun Yaakov Megillah 13b