When is the conclusion of Shabbos and hence Melacha is permitted?
The conclusion of Shabbos which allows one to perform Melacha is dependent on three factors:
- The time of the night.
- Saying Havdala or Baruch Hamavdil.
- Conclusion of Maariv in Shul.
- Some are accustomed to further delay doing Melacha until Havdala or Melaveh Malka.
At what time is Shabbos over?
Shabbos ends at night after three small stars are able to be seen. One must delay doing Melacha until he sees three small stars consecutively in one line. It does not suffice with three small stars which are scattered. [The amount of time between seeing three scattered small stars and three small stars consecutively is four minutes. Hence once three small scattered stars can be seen one is to wait four minutes and may then do Melacha. However see Q&A for other opinions.]
If it is a cloudy day and one cannot see the stars, he is to delay doing Melacha until he knows for certain that the stars are visible. [Likewise if one is in doubt if the stars are small he is to wait until he knows for certain that they are small.]
Melacha during Bein Hashmashos: In all circumstances that Rabbinical Melacha is permitted during Bein Hashmashos, it is permitted by both Bein Hashmashos of Erev Shabbos and Motzei Shabbos. [For a list of circumstance in which Rabbinical Melacha is allowed during Bein Hashmashos-see chapter 261.]
Practically how much time after sunset is the conclusion of Shabbos?
In Eretz Yisrael: One is to wait a minimum of 34-35 minutes after sunset during the summer months, and a minimum of 28-29 minutes after sunset in the winter months. [Others rule one is to wait 32 minutes past sunset in the winter months and 38 minutes past sunset in the summer months. Others rule one is to wait between 45-50 minutes after sunset. ]
Diaspora: The amount of time one needs to wait after sunset fluctuates according to each region. The custom in New York is to wait 50 minutes from sunset. [In general the custom is that each country calculates the leave of Shabbos based on when the sun reaches eight degrees below the horizon.]
Zman Rabbeinu Tam:
It is not the Chabad custom [or the custom of majority of world Jewry] to delay the exit of Shabbos until after the Zman Rabbeinu Tam. Nevertheless many meticulous Jews are accustomed to be stringent in this matter.
How long after sunset is Zman Rabbeinu Tam: Those which follow Rabbeinu Tam wait 72 minutes [or 96 minutes] past sunset in all areas of the world, [besides for those areas in which the time of Geonim is later than Rabbeinu Tam]. There is dispute amongst Poskim if the Zman of Rabbeinu Tam fluctuates between the winter and summer months or remain a literal 72 minutes throughout the year.
The more one delays the end of Shabbos, so is his time in Gehenim delayed:
It is written in the name of Tzadikim that based on how long one delays his leave of Shabbos will be the delay of his leave from Gehenim on Shabbos, as on Shabbos the souls are freed from Gehenem.
What is one to do if he does not know what time Shabbos is over?
If he cannot tell the difference between medium and small stars, then he is to wait until the sky is filled with stars and he may then do Melacha.
Q&A on Zman Rabbeinu Tam
May one who follows the time of Rabbeinu Tam change his custom to follow the earlier time?
Yes although he must first do Hataras Nedarim. If this is the custom of one’s forefathers, he is not to change his custom.
May one who follows Rabbeinu Tam ask another to do Melacha for him?
No.  If however one follows Rabbeinu Tam only regarding Shabbos in order to delay Shabbos as much as possible, while regarding other matters [such as Taanis] he follows the Geonim, then he may ask others to do Melacha for him.
May one who follows Rabbeinu Tam Daven Maariv prior to the leave of Shabbos of Rabbeinu Tam?
Yes. Although some write that if one follows Rabbeinu Tam regarding all matters he is to refrain from doing so unless it is a time of need.
Q&A On Tosefes Shabbos
How does one fulfill the Mitzvah of Tosefes Shabbos?
Upon the conclusion of Shabbos, which is defined as the visibility of three small stars, one fulfils the Mitzvah of Tosefes Shabbos until he says Havdala over wine, or in Maariv, or says Baruch Hamavdil, [or does Melacha]. This applies even if he does not consciously intend to fulfill the Mitzvah, nevertheless so long as he has not said Hamavdil and has not done Melacha his Shabbos is still continuing. There is thus no need to consciously have in one’s mind or verbalize that he desires to continue the Shabbos past nightfall, although some suggest doing so.
How much time of Tosefes Shabbos is one to add after Shabbos?
Once three small scattered stars can be seen one is to wait four minutes and may then do Melacha.
Some however write the custom is to wait fifteen minutes from after three small stars become visible.
Is there a maximum amount of time that one can add to the Mitzvah of Tosefes Shabbos?
Some rule up to three hours. Others rule one can only add to Shabbos up to 72 minutes from after Shabbos. Others rule one can only add a mere moment. Others rule one can add as much as he desires.
 Although night time is generally defined as when three medium stars are able to be seen, nevertheless since people are not expert in the difference of size between a large and medium star, therefore one is required to wait until three small stars are seen. [ibid; Beis Yosef in name of Rabbeinu Yonah]
Biurim: Regarding the fact that in the Shulchan Aruch Admur rules like Rabbeinu Tam that Tzeis Hakochavim begins only 72/96 minutes after nightfall, while here it is omitted and only the sign of stars is mentioned, M”B 293/6 asks this same question on the Michaber. He explains the calculation of stars is only given when one cannot calculate the time, such as he does not know when sunset occurred and he thus cannot calculate the 72 minutes. Ideally however if one could calculate the 72 minutes, he is to delay 72 minutes according to Rabbeinu Tam. However in the Biur Halacha “Ad” he brings from the Minchas Cohen that even according to Rabbeinu Tam once three small stars can be seen there is no need to wait any longer. [See however Piskeiy Teshuvos 293 footnote 21 for those which argue on this conclusion]
 Ideally Shabbos is over once three small stars can be seen, even if they are scattered. However it is a positive command to add onto Shabbos, and this measurement of time is a minimum of waiting until three small stars can be seen in one row. [ibid]
Other Opinions: The Digul Merivav rules it suffices to wait a slight amount of time after one sees three small stars, even if they are scattered. Others hold one is to wait until the entire sky is filled with stars. [See Kaf Hachaim 293/4]
 Ketzos Hashulchan 93 footnote 1. It is not mentioned in Poskim the definition of consecutive stars. Thus one is to wait four minutes, as the reason for waiting to see three small consecutive stars is for Tosefes Shabbos, and since 4 minutes is the minimum amount of time needed for Tosefes Shabbos, therefore once three small stars are visible one is to wait four minutes and do Melacha.
Other Opinions: The above ruling is based on the understanding of the Ketzos Hashulchan ibid and Avnei Nezer 498/3 in the Seder Hachnasas Shabbos of Admur, that the time of Tosefes Shabbos is 4 minutes. Eretz Tzevi 60 however negates their proofs and rules one cannot learn from the above area in Admur anything with regards to the minimum Shiur of time, and rather even waiting one moment is valid.
 Ketzos Hashulchan 93 footnote 2
 Although there is an opinion [M”A 342] which doubts whether the allowances of Bein Hashmashos of Erev Shabbos apply equally to Bein Hashmashos of Motzei Shabbos, as perhaps due to the doubt of Bein Hashmashos the holiness of the essence of the day of Shabbos does not leave, (nevertheless one may be lenient). [ibid] So rules also Bircheiy Yosef and Beis Meir that there is no differentiation. [M”B 342/2], and so seems to be the opinion of the M”A himself in 623/4 which allows blowing Shofar by Bein Hashmashos of Yom Kippur, as explains the Peri Megadim there.
Other Opinions: The Chayeh Adam [brought in M”B 342/2 concludes that one is to be stringent.
Opinion of the Mishneh Berurah: In Biur Halacha 342 “Bein Hashmashos” he concludes to be stringent by Bein Hashmashos of Motzei Shabbos as in addition to the doubts of the M”A ibid, there are also many doubts involved with when is Bein Hashmashos.
 The first calculation brought [not in brackets] is based on Ketzos Hashulchan 93 footnote 2; Shiureiy Tziyon of Grach Naah p. 76 and Kitzur Halachos Shabbos Miluim p. 92-100. This calculation follows the amount of minutes needed to wait after sunset as ruled by the Ketzos Hashulchan, and includes the novelty of Admur that sunset begins only 4  minutes after the visible sunset which is recorded on calendars.
Biurim: Regarding the fact that at the mentioned times small stars are not yet visible in the sky, see M”B 293/6; Kitzur Hilchos Shabbos Miluim p. 100-101 for an explanation on this matter. There he explains that in truth the stars are visible under proper weather conditions, and that the main calculation is based on time, and the calculation of stars is only given when one cannot calculate the time, such as he does not know when sunset occurred.
 See Kitzur Hilchos Shabbos Miluim p. 99 that in Eretz Yisrael there are 5 minutes between the Shekia Haniris and the Shekia Haamitis. Thus we wrote between 34-35 minutes taking into account this extra minute.
 This refers to Shekia Haniris, which is the sunset written in calendars.
Shekia Haniris “the visible sunset” is 4/5 minutes before the Shekia Hamitis “the true sunset”. Hence from the true sunset one must only wait 20/21 minutes in the winter and 26/27 minutes in the summer. [Kitzur Hilchos Shabbos 293 based on Admur in Seder Hachnasas Shabbos]
 Bein Hashmashos in Eretz Yisrael varies between the summer and winter months.
 Background of above calculation:
- Sunset begins 4-5 minutes after the visible sunset recorded on calendars. [See Seder Hachnasas Shabbos of Admur, and Kitzur Halachos ibid]
- After 4-5 pass and sunset begins Bein Hashmashos lasts between 15-26 minutes depending on the time of year. It lasts between 15-20minutes in the winter and 20-26 minutes in the summer. [See Ketzos Hashulchan and Shiureiy Tziyon ibid]
- One is to add 4 minutes for Tosefes Shabbos.
Grand Total: Based on the above calculation in order to be sure it is always past the amount of time needed to wait one is to always wait during the winter months a minimum of 29 minutes after the recorded sunset in calendars and in the summer he is to wait 35 minutes from the recorded time in the calendar.
Ruling of Harav Hagaon Avraham Chaim Naah in Ketzos Hashulchan ibid and Shiureiy Tziyon ibid:
One is to wait a minimum of 30 minutes after sunset during the summer months [Nissan-Tishreiy], and a minimum of 24 minutes after sunset in the winter months [Tishreiy-Nissan].
VeTzaruch Iyun in Ketzos Hashulchan if this refers to the Shekia Hamitis or Shekia Haniris which is a 4 minute difference. No mention is made what Shekia he is referring to neither in Ketzos Hashulchan or in Shiureiy Tziyon. Kitzur Halachos Admur Miluim p. 104 learns in the Ketzos Hashulchan that we measure from the Shekia Haniris. Although he himself argues on this premises concluding it must refer to the Shekia Haamitis.
 Sefer Bein Hashmashos of Rav Yechial Michel Tuchensky brought in Piskeiy Teshuvos 293/4. There he also records the opinion of Grach Naah which we wrote above.
 They checked and found that three small consecutive stars cannot be seen prior to this time in summer and winter. At this time the sun is 8 degrees below the horizon.
 Piskeiy Teshuvos ibid in name of Chazon Ish and Rav Aron of Belz
 Igros Moshe 4/62; However the Rebbe [Shaareiy Halacha Uminhag 155] after bringing the custom expressed himself there “I am not responsible for this calculation.” See there that some wanted to suggest that based on Admur one must always wait 72 minutes. However the Rebbe negated this logic.
 See Piskeiy Teshuvos 293/4; Sefer Biur Halacha [Zilber] 293
 Ketzos Hashulchan 93 footnote 2; See Piskeiy Teshuvos 293/4 for a summary of all the opinions regarding Rabbeinu Tam after Shabbos.
 Piskeiy Teshuvos ibid
 Ketzos Hashulchan 93 footnote 2
 See 261/5. However based on the calculation of 24 minutes per Mil, Zman Rabbeinu Tam is 96 minutes after sunset, and so is the custom of some communities. [Beis Avi 3/117; Beis Yisrael 1/180; Piskeiy Teshuvos ibid]
 So is the custom of some communities as mentioned in the Poskim in previous footnote.
 It is interesting to note that in many European countries [and Russia] the time of Rabbeinu Tam is very close in time to the time of the exit of Shabbos which we follow. [sometimes even only one minute apart] Absurdly in certain Northeast countries the time of Rabbeinu Tam actually falls earlier than the time we follow based on the Geonim!
 See Piskeiy Teshuvos 256 footnote 12
 Piskeiy Teshuvos 293/5
 Piskeiy Teshuvos 293/4
 Sheivet Haleivi 1/53; Minchas Shabbos 263/7; Piskeiy Teshuvos 263/44 and 293
 Teshuvos Vehanhagos 1/234
 See Piskeiy Teshuvos 293/6
 Divreiy Yatziv 1/1134; However see SSH”K 59 footnote 17 which is lenient even in such a case.
 See Piskeiy Teshuvos 293/2 footnote 5
 Seemingly once one does Melacha, even if it is prior to saying Hamavdil, his Tosefes Shabbos is now over, as this action itself proves that he has now ended his Tosefes Shabbos. We do not say that it is still Tosefes Shabbos and he has thus done a prohibition of Melacha during his Tosefes Shabbos. [Accordingly this would answer the questions posed in Ashel Avraham Butchacher 299] It is nevertheless forbidden to do Melacha until one says Hamavdil due to the obligation to escort the king with Hamavdil prior to Melacha. See next Halacha. [So writes also Piskeiy Teshuvos 293/2 footnote 10]
 Piskeiy Teshuvos ibid suggest doing so, even though this concept is not brought anywhere in Poskim. In Mishmeres Shalom 29/6 he brings that one is to verbalize his acceptance of Tosefes Shabbos, although this is with regards to Erev Shabbos and not Motzei Shabbos as is implied from there.
 Ketzos Hashulchan 93 footnote 1. It is not mentioned in Poskim the definition of consecutive stars. Thus one is to wait four minutes as the reason for waiting to see three small consecutive stars is for Tosefes Shabbos, and since 4 minutes is the minimum amount of time needed for Tosefes Shabbos, therefore once three small stars are visible one is to wait four minutes and do Melacha.
Other Opinions: The above ruling is based on the understanding of the Ketzos Hashulchan and Avnei Nezer 498/3 in the Seder Hachnasas Shabbos of Admur, that the time of Tosefes Shabbos is 4 minutes. Eretz Tzevi 60 however negates their proofs and rules one cannot learn from the above area in Admur anything with regards to the minimum Shiur of time, and rather even waiting one moment is valid.
 Toras Shabbos 293/1; and so writes Piskeiy Teshuvos 293/2
 They write there that it is the “custom of Israel”.
 Piskeiy Teshuvos 293/2
 This does not mean to ask “must one must break Shabbos after a maximum amount of time” but rather until how long does he fulfill the Mitzvah of Tosefes Shabbos.
 Shulchan Hatahor 188/15
 Olas Tamid brought in M”A 188/18
 Peri Megadim 188 A”A 18 in his explanation of M”A that after one moment of Tosefes Shabbos, it is now over.
 Piskeiy Teshuvos 293/2 based on Peri Megadim and Bach
Regarding the question of how long one can extend Shabbos – Certainly it is a dovor poshut that one must end Shabbos with enough time to daven Maariv [make Havdalah] and eat melava malka before chatzos, as these mitzvos [Maariv and melava malka] are required l’chatchila to be done before chatzos. Vetzurich iyun as to how one can add “as much time as he desires” if doing so will conflict with his obligation to carry out the above mitzvos before chatzos. To note, however, that the Amshinover Rebbe keeps Shabbos until Tuesday.