The Gemara, Yoma 81b, states that there is a mitzvah to accept Yom Kippur upon oneself prior to the actual time of its arrival. This mitzvah is known as tosefet Yom HaKippurim. The Gemara states that this mitzvah applies to Shabbat and Yom Tov as well. Rambam, Hilchot Shevitat Asor 1:6, codifies the mitzvah of tosefet Yom HaKippurim but does not codify the mitzvah of tosefet Shabbat or tosefet Yom Tov. Magid Mishna, ad loc., notes that Rambam is of the opinion that there are other opinions in the Gemara that do not subscribe to the concept of tosefet Shabbat and tosefet Yom Tov, and Rambam follows those opinions. Nevertheless, most Rishonim (see Beit Yosef, Orach Chaim 261) are of the opinion that there is a mitzvah of tosefet Shabbat and tosefet Yom Tov. As such, Shulchan Aruch 263:2, writes that one must accept Shabbat upon oneself prior to sundown.

Rambam is not only of the opinion that there is no mitzvah of tosefet Shabbat, he does not even recognize accepting Shabbat early as a valid halachic mechanism. Despite his position, Rambam, Hilchot Shabbat 29:11, still allows one to recite kiddush before the actual start of Shabbat. Rambam explains that kiddush does not have to be recited on Shabbat, it may also be recited prior to Shabbat. Although Rambam's opinion regarding the concept of tosefet Shabbat (or lack thereof) is rejected, Rambam's explanation of how it is possible to recite kiddush before Shabbat will serve as an explanation for the more normative opinions of tosefet Shabbat as will be explained below.

The Scope of Tosefet Shabbat
R. Yosef D. Soloveitchik, in Nefesh HaRav pg. 155, presents two approaches to the concept of tosefet Shabbat. One can understand tosefet Shabbat as a limitation. By accepting Shabbat early, one prohibits oneself from any melacha that is prohibited on Shabbat. Alternatively, one can understand that tosefet Shabbat creates kedushat Shabbat (the sanctity of Shabbat) and by accepting Shabbat early one actually extends Shabbat.

R. Ya'akov B. Zolty, Mishnat Ya'avetz, Orach Chaim no. 29, makes a similar presentation, and notes that there is a practical difference regarding whether one may recite kiddush during the time of tosefet Shabbat. If one assumes that tosefet Shabbat is merely a limitation in which one prohibits oneself from melacha, one may not recite kiddush during this time. However, if tosefet Shabbat creates kedushat Shabbat, one may recite kiddush during this time. Nevertheless, R. Zolty suggests that even if one assumes tosefet Shabbat to be limited in scope, one may still recite kiddush during this time based on the opinion of Rambam. As mentioned previously, Rambam allows recitation of kiddush even prior to Shabbat. Therefore, it makes no difference whether tosefet Shabbat has the actual status of Shabbat, one may recite kiddush during this time.

R. Zolty adds that within the approach that tosefet Shabbat produces a full kedushat Shabbat, one can further question whether tosefet Shabbat imposes kedushat Shabbat onto Friday afternoon, or whether tosefet Shabbat transforms Friday afternoon into Shabbat. A practical application to this question lies in a discussion regarding a fast day that occurs on a Friday. Maharam (cited in Mordechai, Eruvin no. 494) is of the opinion that if one accepts Shabbat early on a fast day, one may eat before the actual conclusion of the fast. Ra'aviah no. 858, disagrees and maintains that one must wait until the actual conclusion of the fast. Apparently Maharam is of the opinion that tosefet Shabbat transforms Friday afternoon into Shabbat. Therefore, once one accepts Shabbat, it is no longer Friday and the fast is complete. Ra'aviah is of the opinion that tosefet Shabbat merely imposes kedushat Shabbat onto Friday afternoon. Although one is observing Shabbat, one must concurrently observe the laws that relate to the actual day and therefore may not eat until the completion of that day. Rama, Orach Chaim 249:4, rules that one may rely on the opinion of Maharam for a private fast day, but one must be stringent on a public fast day. [See also Rama, Yoreh Deah 196:1, regarding another dispute that relates to this question.]

Bach, Orach Chaim 472, quotes the opinion of Maharal that one who accepts Shabbat early cannot fulfill the mitzvah of eating the Shabbat meal until nightfall. He claims that the three Shabbat meals must be eaten on the actual day of Shabbat. Taz, Orach Chaim 491:6, disagrees and contends that there should be no difference between eating the meal during the time of tosefet Shabbat, and eating the meal during the actual day of Shabbat. R. Zolty explains that Bach is of the opinion that tosefet Shabbat merely imposes kedushat Shabbat onto Friday afternoon. Therefore, if one assumes that the Shabbat meals must be eaten on the actual day of Shabbat, tosefet Shabbat would be excluded. However, Taz is of the opinion that tosefet Shabbat transforms Friday afternoon into Shabbat. Therefore, tosefet Shabbat is considered the actual day of Shabbat, and there can be no specific requirement that the meal be eaten after nightfall. Mishna Berurah 267:5, writes that ideally one should follow the opinion of Bach and eat a k'zayit of bread after nightfall.

The Proper Time For Tosefet Shabbat
Rosh, Berachot 4:6, writes that one may not accept Shabbat before plag hamincha (one and one quarter halachic hours before the end of the day). Shulchan Aruch, Orach Chaim 267:1, codifies Rosh's opinion as normative. Mishna Berurah 263:18, rules that if one accepts Shabbat before plag hamincha, the acceptance is invalid.

There is a dispute among the Rishonim regarding the latest time one may accept Shabbat. Tosafot, Beitzah 30a, s.v. De'ha, note that tosefet Shabbat must be a significant amount of time prior to sundown. However, Tosafot do not know exactly how much time is necessary in order to fulfill the mitzvah of tosefet Shabbat. Ran, Shabbat 15a, s.v. Amar quotes Ramban that one can even fulfill tosefet Shabbat by accepting Shabbat a short amount of time before it is actually prohibited to perform melacha. Shita Mekubetzet, Beitzah 30a, s.v. VeLo quotes Rabbeinu Yitzchak that one should accept tosefet Shabbat one-half hour prior to sundown.

Shulchan Aruch, Orach Chaim 261:1, rules in accordance with the opinion of Ramban that one can fulfill the mitzvah of tosefet Shabbat by accepting Shabbat a short amount of time prior to Shabbat. Mishna Berurah 261:23, (based on several considerations) recommends that one accept Shabbat one-half hour prior to sundown. If that is not possible, he suggests that one accept Shabbat twenty minutes prior to sundown.

Both R. Moshe Feinstein and R. Shlomo Z. Auerbach suggest that there may be a difference between women and men regarding the amount of time set aside for tosefet Shabbat. R. Feinstein, Igrot Moshe, Orach Chaim 2:6, notes that the traditional candle lighting time of eighteen minutes prior to sundown is based on a minhag that women accepted upon themselves to keep eighteen minutes of tosefet Shabbat. R. Feinstein implies that men can accept Shabbat a few minutes later. [See however, Igrot Moshe, Orach Chaim 4:62, where R. Feinstein implies that everyone should accept Shabbat at least eighteen minutes prior to sundown.] Similarly, there is a minhag in Jerusalem to accept Shabbat forty minutes prior to sundown. R. Auerbach, in Shemirat Shabbat Kehilchata ch. 46, note 20, suggests that the minhag applies to women, and not to men.