After discussing Yom Kippur, the Torah states, “Shabbat Shabbaton Hu Lachem VeInitem Et Nafshoteichem BTish’ah LaChodesh BaErev MeiErev Ad Erev, Tishbitu Shabbatchem,” “It is a Shabbat of complete rest for you, and you shall afflict yourselves on the ninth day of the month at evening; from evening to evening you shall rest, on your day of rest” (VaYikra 23:32). Earlier (16:29), it is stated that Yom Kippur is observed on the tenth day of the month, but here it is stated that Yom Kippur begins during the evening on the ninth day of the month. The Gemara (Yoma 81b) uses this Pasuk to derive the obligation of adding to both the start and end of Yom Kippur, and the Gemara extends this obligation to include Shabbat and all other holidays.
Most Rishonim explain that the obligation of Tosefet Shabbat, adding onto Shabbat, is mandated by the Torah. However, Rambam disagrees and explains that only Yom Kippur is biblically mandated. The Magid Mishneh and Kesef Mishneh differ as to whether the Rambam believes that Tosefet Shabbat is required only on a rabbinic level or whether the requirement is non-existent. There is a rather simple explanation for the Rishonim who state that Tosefet Shabbat does not exist. Since the underlying reason behind the Mitzvah of Shabbat is to commemoratethe creation of the world, one should not add to Shabbat, just like Hashem did not add to the creation of the world on Shabbat. However, if Shabbat primarily commemorates Yetzi’at Mitzrayim, then Shabbat is just a day of rest, and there would be no problem adding to it.
Tosefet Shabbat can be looked at in two ways. The first is that it is just a precautionary action taken to avoid doing Melachah on Shabbat, and the second is that Tosefet Shabbat actually has a fundamental value. Rashi (BeReishit 2:2 s.v VaYechal Elokim BaYom HaShevi’i) states that people that are not precise with time must add onto Shabbat as a protective measure, whereas the Ran (Beitzah 30a) holds that one should add onto Shabbat even more than one would for just precautionary purposes. We can infer from this that the Ran believes Tosefet Shabbat has intrinsic value.
There are many different opinions as to how much time must be added to Shabbat. The two most common answers are given by the Mishnah Berurah and Rav Moshe Feinstein along with Rav Ovadiah Yosef. The Mishnah Berurah writes (261:23) that one should accept Shabbat twenty minutes prior to sunset. However, Rav Moshe (Teshuvot Igrot Moshe Orach Chaim 1:96) and Rav Yosef (Teshuvot Yabia Omer 5: Orach Chaim 21) are of the opinion that one may add any amount of time, even just a few minutes.
There is a story told by a student of the Chafetz Chaim that illustrates the enormity of the Mitzvah of Tosefet Shabbat. There was once a child who became gravely ill, and the physicians were unable to cure him. In desperation, the parents approached the Chafetz Chaim for guidance. He urged them to begin Shabbat early each week, ensuring to have the table set and the candles lit well before the twenty minute period prior to sundown. The couple followed his advice, and the child was miraculously cured. This story illustrates how impactful it can be to add just a few minutes onto Shabbat. By realizing the importance of every minute of Shabbat, we will hopefully be able to observe it to its fullest extent and make every second of it meaningful.