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Not before Tzeis: One does not fulfill the Mitzvah of eating Matzah prior to nightfall [i.e. Tzeis Hakochavim].
Before midnight: One is required to fulfill the Mitzvah of eating Matzah prior to midnight. [If one eats the Matzah only after midnight, some Poskim rule he does not fulfill his obligation and has thus lost the Mitzvah. Other Poskim rule he can nevertheless fulfill the Biblical Mitzvah up until Alos Hashachar. Other Poskim rule that this matter is left in question, and hence one is to be stringent like both opinions, and eat Matzah without a blessing. Practically, if one did not yet eat the Matzah and it is already after midnight, he is to eat the Matzah without the blessing of “Al Achilas Matzah.” If it is already close to midnight and one did not yet eat the Matzah, he is to skip from wherever he is holding and perform Motzie Matzah before midnight. The same applies to a person who awoke from sleep, moments before midnight that he should say Kadesh and immediately afterwards recite Motzie Matzah. This applies for both the first and second Seder.]
On the first night of Pesach one is Biblically obligated to eat a Kezayis of Matzah [within Kdei Achilas Peras]. [Likewise, in order to be obligated to recite Birchas Hamazon throughout Pesach, one must eat a Kezayis of Matzah within Achilas Peras.]
How much is a Kezayis? It is disputed amongst Poskim as to whether the Kezayis is ½ of a Kebeitza or 1/3 of a Kebeitza. Practically, regarding the Biblical Mitzvah of eating Matzah, and regarding Safek Brachos, we are stringent like the opinion that it contains ½ of a Beitza. This is measured in volume and not weight. [This amounts to 26cc in volume. Some Poskim rule that this amounts to 28.8 grams of Matzah. However, a more thorough measurement of the amount of crushed Matzah that can fit in a 30cc volume cup, reaches only 20 grams. Thus, while it is accustomed to eat close to thirty grams of Matzah for the Biblical Kezayis, one may be lenient with 20 grams if he so chooses.]
Within how much time is the Matzah to be eaten?
The entire Kezayis of Matzah must be eaten within the amount of time called “Kdei Achilas Peras.” [This is approximately within 3-4 minutes.]
Intending to fulfill the Mitzvah:
Upon eating the Matzah on the night of the 15th, one is to have in mind to fulfill the Biblical command of eating Matzah. One is to publicize this matter to the attendants of the Seder, especially if they have little Jewish background. If one ate Matzah without intent to fulfill the Mitzvah, he nevertheless fulfills his obligation so long as one knows of the Mitzvah to eat Matzah on Pesach, and knows that the night is Pesach.
 Admur 472:2
 Admur 458:2 “One is required to eat the Matzah prior to midnight, just like the law regarding the Karban Pesach which may only be eaten until midnight,”; Admur 639:20 “And one is to beware to eat in the Sukkah prior to midnight, just as is required regarding eating Matzah”; 477:6 regarding Afikoman; See 472:2 regarding the Hekesh being used to obligate eating Matzah after Tzeis Hakochavim [Vetzaruch Iyun why the end time was not mentioned in this Hekesh, and why we make the Hekesh for the end time only regarding Matzah and not the four cups, also Tzaruch Iyun why in 475:20-23 no mention of Chatzos was made; See here: http:::shulchanharav.com:text.asp?index=128712&arc_id=138425]; Michaber 477:1 and Beis Yosef 477 regarding Afikoman; Degul Merivava 477; Shaareiy Teshuvah 477:1; M”B 477:6 “One must be very careful”; Biur Halacha 477; Kaf Hachaim 477; Piskeiy Teshuvos
 The reason: As just like the Karban Pesach may only be eaten until midnight, so too the Matzah may only be eaten until midnight, as the verse states Al Matzos Umerorim Yochluhu, thus teaching us that the eating of Matzah is connected, and follows similar laws, to the Pesach [Admur 458:2] This follows the opinion of Rebbe Elazar Ben Azaria, brought next.
 Reb Elazar Ben Azaria in Pesachim 120b; Zevachim 57b; Tosafos Zevachim ibid; Megillah 21a; Piskeiy Tosafos Pesachim 334; Zevachim 33; Rabbeinu Chananel in Arvei Pesachim; Semag in name of Riy; Mordechai end of Pesachim; Rabbeinu Yerucham 5:4; Conclusion of Or Zarua; Hapardes 9:16-26 “It is forbidden to eat Matzah after midnight”; See Biur Halacha 477:1 “Viyihei Zahir”
 Rebbe Akiva in Pesachim and Zevachim ibid; Rambam Karban Pesach 8:15; Chametz Umatzah 6; Rif [brought in Mishkanos Yaakov 139] Rav Hamaggid; Baal Haitur; Or Zarua in name of Rav Yaakov of Krubil; Kol Bo 90; See Biur Halacha 477:1 “Viyihei Zahir”
 Rosh; Rashba; Ran end of Megillah chapter 2; Rokeiach
 Degul Merivava 477; Shaareiy Teshuvah 477:1; P”M 477 M”Z 1; Derech Hachaim; Tov Ayin 38:87; Ikarei Hadat 19:14; Misgeres Hashulchan 119:11; Aruch Hashulchan 477:5; M”B 477:6; Biur Halacha ibid; Kaf Hachaim 473:16; 475:64; Beis Halevi 1:34; Nitei Gavriel 91:1
Other opinions: Some Poskim rule that the final ruling of the Talmud is like Rebbe Akiva that the Mitzvah of Matzah applies throughout the night, until Alos Hashachar. [Mishkanos Yaakov 139; Or Yisrael 475; Or Sameiach 6:1; Sdei Chemed Mareches Chametz Umatzah 14:11; See Nitei Gavriel ibid footnote 1]
 Degul Merivava 477; Shaareiy Teshuvah ibid; M”B 477:6; Ashel Avraham Butchach 477; Kaf Hachaim 477:10
 Admur 481:1 “There is no difference between the first and second night in any other matter”; Nitei Gavriel 91:5
Other opinions: Some Poskim defend the practice of those who recite a blessing over the Matzah even after midday. [Darkei Chaim Veshalom 605; See Nitei Gavriel ibid footnote 6]
 Admur 475:32; Michaber 475:5; Rambam 6:1
 Admur 486:1
 Admur 486:2
 Shiurei Torah 3:13
 Shiurei Torah 3:13 [p. 184 and 191; Vetzaruch Iyun as explained next
 Heard from Rav Eli Landau Shlita based on his measurement; Koveitz Beis Ahron Viyisrael 51:86; See also Shiureiy Torah in footnote 18 who writes that he managed to enter 20 grams of crushed Matzah in a 35 cc cup which would make a Kezayis be only 17 grams.
 Admur 475:8 and 14; Michaber 475:6; Pesachim 114b
 Four minutes: Shiurei Torah 3:15 [p. 303]; Aruch Hashulchan 202:8; Kaf Hachaim 210:5; Piskeiy Teshuvos 210:1 that so is the widespread custom
Three minutes-Chabad custom: See Sefer Haminhagim ibid and footnotes 342-344 for a quote of various opinions of Achilas Peras, and for a tradition from the Tzemach Tzedek, in name of Rav Hillel Miparitch, that differs from the response of the Tzemach Tzedek which placed Achilas Peras as 6-7 minutes and rather places Achilas Peras as minimum 3 minutes and maximum 7 minutes. The Rebbe concludes there in the footnote that one is required to suspect for the above tradition. So also rules Igros Moshe 4:41 that Achilas Peras is 3 minutes regarding Bracha Achrona. Accordingly, one should try to eat the Kezayis within three minutes. However, if one ate it within four minutes, and perhaps even within 6-7 minutes, one can rely on the Tzemach Tzedek’s other rulings. Vetzaruch Iyun, as perhaps Safek Brachos Lihakel. In any event one is to be very careful in the above.
Other opinions: See Shiurei Torah 3:15; Sefer Haminhagim ibid and footnotes 342-344; Piskeiy Teshuvos 158:7 and 210:1; 475:10
 See Admur 475:28-29