Although Yom Kippur is observed on the 10th of Tishrei, the Torah makes reference to fasting on the 9th. The Talmud (Rosh HaShanah 9a) interprets this as a reference to the obligation of adding on from erev Yom Kippur to Yom Kippur, i.e. beginning the fast while it is still the 9th. Most rishonim interpret this as a biblical obligation to bring every Shabbat and Yom Tov in early. A notable exception is the Rambam (Hil Shvitat haAsor 1:6) who takes a minimalist view of the Talmudic interpretation, quoting an obligation only in reference to Yom Kippur and only in reference to fasting. Later authorities debated whether the Rambam recognized Tosefet Shabbat as a rabbinical obligation (See Magid Mishnah, and Meshekh Chokhmah, Vayikra 23:21), or denied it completely (see Kessef Mishneh Hil. Shabbat 5:1 and Resp. Avnei Nezer, O.C, 498:4).

Two perspectives appear to exist as to what is accomplished by Tosefet Shabbat. One perspective is that upon early acceptance of Shabbat, Shabbat actually begins for all intents and purposes. Another perspective is that Shabbat can not actually be started early; rather, Tosefet means that the individual accepts the prohibition of melakhah early, even though it is not yet Shabbat. This second possibility might be a result of a desire to avoid violating Shabbat by coming too close to the actual starting time (see Bereishit Rabbah, ch. 10; Shiltei Giborim Yoma ch. 8 #2, Lekach Tov 8:5; although contrast Ran, ch. 2 of Beitzah).

Several issues are dependant on this question, including: a) the possibility of having the Friday night Shabbat meal during the time of tosefet (see Tos. Pesachim 99b, and Taz, O.C. 291:6, citing Maharshal); b) the possibility of getting married during tosefet Yom Tov, despite the prohibition of getting married on Yom Tov itself (see Tos. Ketubot 47a s.v. d’masar); c) the possibility of partial acceptance of Shabbat (see Emek Brakhah, p. 66; Resp. Har Tzvi, O.C. 139; Resp. Shevet HaLevi, I, 107). d) the view of the Magen Avraham and the Taz (O.C. 494) that one should not bring in Shavuot early, as that would compromise the “completeness” (temimut) of the seven weeks of sefirah; e) the question of one who brings in Yom Tov and then learns of an obligation to begin mourning practices – does that now wait until after Yom Tov, when it would be observed in full, or does one have the ability to begin mourning immediately, and allow the actual onset of Yom Tov to cancel the mourning period (see Resp. Meshiv Davar, Y.D. 73, and Resp. Teshuvah Sh’leimah, I, Y.D. 46); f) the status of the product of melakhah done during the time of tosefet (see Resp. Kinyan Torah B’Halakhah, VII, 21); g) if accepting Shabbat is susceptible to being abrogated by a rabbinical scholar (see Elyah Rabbah 263:38, and Levush 263:17, and Taz, O.C 600).

One major issue that must be dealt with separately, though, is the question of Kiddush. The Talmud states explicitly (Berakhot 27b) that Kiddush may be recited while it is still daytime. This is difficult if tosefet is not understood to mean actual Shabbat; particularly so for the Rambam, who either recognizes tosefet only as a rabbinical concept (and Kiddush is a biblical obligation), or not at all. If it is the former possibility, the position of Rabbeinu Tuvyah of Vienna may be helpful; his view (cited by the Mordechai, Megilah ch. 2, and Magen Avraham, O.C., 267:1) is that a biblical obligation can be fulfilled during a time period that is rabbinical in nature but will become biblical. However, if the Rambam is understood as not recognizing tosefet at all, a different explanation is required. That explanation would seem to be contained in the fact that the Rambam does not require Kiddush to be recited on Shabbat, merely close to Shabbat (Hil. Shabbat 29:11). R. Herschel Schachter suggests this may be because Kiddush might serve as an announcement concerned the abstention from labor, that it is intentional and not merely idleness (as he writes concerning muktzeh, Hil. Shabbat 24:13). Thus, this purpose is served when Kiddush is recited in close proximity to the onset of Shabbat.