1) The Fourth Commandment is to “Remember (Zachor) the Shabbos day”. Our Sages in the Mechilta explain that derived from this verse is the obligation of Tosefes Shabbos, starting the Shabbos early in order to incorporate a small part of the week day into Shabbos. The majority of the Rishonim maintain that the mitzvah of Tosefes Shabbos is a Biblical commandment. (See Biur Halacha 261:2) One should perform this mitzvah before sunset, since Shabbos begins at sunset.
2) This Mitzvah is incumbent upon women as well as men. (Shemirash Shabbos K’Hilchoso 46:1)
3) The proper procedure, lechatchila, for this mitzvah, is to state that one is being mekabel Shabbos for the sake of the mitzvah (Tosefes Shabbos). This statement can be made in one of the following ways: by reciting the entire kabbalas Shabbos, by reciting Mizmor Shir L’yom Hashabbos, by answering Borchu, by davening the Shabbos Maariv, by lighting candles [for women], or even by saying aloud: I am mekabel Shabbos for the sake of the mitzvah of Tosefes Shabbos. If one merely thought to be mekabel Shabbos, without verbalizing anything, according to many poskim he is yotzei b’dieved. (Shemiras Shabbos K’Hilchoso 46:2) There are some poskim who say that one may fulfill the obligation by merely refraining from forbidden work (see Aruch Hashulchan 261:2 and Yabia Omer vol. 7 page 97 in the footnote) ; however, many poskim disagree. (Shemiras Shabbos K’Hilchoso ibid.)
4) The Rishonim do not clearly define the amount of time as the minimum addition required to fulfill the mitzvah of Tosefes Shabbos. However, latter day poskim do define specific amounts of time to fulfill the mitzvah: The Chayei Adam (5:2), cited by the Mishna Berurah (261:22), maintains that Tosefes Shabbos together with Bein Hashmashos is one half hour long. Because Bain Hashmashos is approximately fifteen minutes, one should add an extra 15 minutes to Shabbos. Rav Moshe Feinstein zt”l, (Igros Moshe Orach Chaim 97), implies that one need not be mekabel Shabbos more than two or three minutes early in oder to fulfill the mitzvah of Tosefes Shabbos. The Mishna Berurah advises that to fulfill the mitzvah according to all the shitos, (most notably the Yeraim), one should usher in Shabbos 30 minutes early, or at least 20 minutes early. Rav Avigdor Nevenzhal shlit”a (B’Yitzchak Yikareh on Mishnah Berurah) cites Rav Shlomo Zalman Aurbach zt”l who says that the custom is not to follow this stringent view of the Mishnah Berurah.
5) One may be mekabel Shabbos as early as plag hamincha, but not earlier. Any Kabbalas Shabbos made before plag hamincha, including lighting candles, is void and must be repeated. (Mishna Berurah 261:25)
6) A common problem which many people are unaware of is that once one answers borchu with a minyan one has accepted upon himself Shabbos and cannot afterwards pray the weekday mincha. If this occurs, he must daven Maariv twice. (S.A. 261:16) If one is in a Shul during the summer months when there is an early minyan taking place, one should not answer to Barchu if he still wishes to daven Mincha later. Rav Moshe Feinstein zt”l (Igros Moshe O.C. 3:37) writes that if one answered Borchu with the specific intent not to accept Shabbos, then he may daven Mincha afterwards. If one does not have this intent it is considered as if one intended to accept Shabbos. A similar view was expressed by Harav Shlomo Zalman Auerbach zt”l in the Sefer Halichos Shlomo (vol. 1 page 166). Harav Ovadia Yosef zt”l (Yechava Daas 6:18) disagrees, and rules that even if one had the specific intent not to accept Shabbos, he may not daven Mincha afterwards. The reason for this rule is that saying Borchu is an act of accepting Shabbos with a congregation and is effective regardless of intent. Therefore, one should not answer Borchu at all.
7) The Sefer Shemiras Shabbos K’Hilchasa (46 footnote 54), adds that even if one did not answer borchu, but turned around to face the back of the Shul during Boei Beshalom, (as is customary), he has accepted Shabbos, and is therefore disqualified from praying Mincha. He would also have to daven Maariv twice. See. however. Ishei Yisroel page 372 note 12 who writes that this ruling of the Shemiras Shabbos is debatable.
8) According to our custom, Shabbos begins at sunset. Once the sun sets it may already be night according to many opinions. Therefore, in order to perform the mitzvah of adding to the Shabbos, one must be mekabel Shabbos before sunset. (After sunset, one is not adding to the Shabbos since it is already Shabbos.) Many Shuls, especially during the winter, daven Mincha on Friday evening 10 to 15 minutes before sunset and then begin the prayer of Kabbalas Shabbos. By the time the minyan says boei beshalom (a formal way of accepting early shabbos), it may be past sunset. Therefore, many men are not fulfilling the mitzvah of tosefes Shabbos according to the opinion of many poskim. There are, however, a couple of alternatives to fulfill the mitzvah and daven Mincha in the proper time:
- If an individual realizes that he will not be able to finish the silent Shemoneh Esrei of Micha before sunset, he should daven alone (byichidus) before sunset, be mekabel Shabbos, and then go to shul to answer kedusha etc. (Shemiras Shabbos K’Hilachasa 46:5).
- Rav Shlomo Zalman Aurbach zt”l is cited (Sefer Halichos Shlomo chapter 14 note 3) as offering an alternative solution to the problem. He advises that if one will not be able to finish the silent Shemoneh Esrei before sunset, one may accept Shabbos before mincha, with the stipulation, “Even though I am accepting Shabbos upon myself, I am still able to daven mincha”. One should add the next phrase to fulfill all the shitos by saying, “Right before sunset I accept Shabbos completely without any stipulation.” See Halichos Shlomo for further discussion on this psak.
- The Tzitz Eliezer wites (13:42) that if one finds himself right before sunset and is faced with the choice of davening Mincha or being mekabel Shabbos, he should be mekabel Shabbos and then daven Mincha. He reasons that the rule that one who accepts Shabbos may no longer daven Mincha is only true when one accepted Shabbos with a minyan (by answering Borchu and the like). However, one who personally accepted Shabbos without a minyan may continue to daven mincha.