For all Torah matters, night precedes day, so Yom Kippur begins at night and concludes at the end of the following day. However, it is unclear as to precisely when day ends and night begins. Does the day end when the sun sets and is no longer visible, or when it gets dark enough to see three medium-sized stars? In Eretz Yisrael, the difference between these times is about twenty minutes, with some seasonal variation. The twilight period between shki’a (sunset) and tzeit ha-kokhavim (nightfall) is referred to as bein ha-shemashot. Since the mitzva to refrain from work on Shabbat and Yom Tov is of Torah origin, we are stringent about the timing, in accordance with the well-known principle, “We are stringent in cases of uncertainty about Torah law.” Thus, Shabbat and Yom Tov begin at shki’a and end at tzeit.
There is also a mitzva to extend Shabbat and Yom Tov, meaning to accept the sanctity of the day a little before its starting time and extend it a little past its ending time. Accordingly, it is a mitzva to accept the day’s sanctity a few minutes before shki’a, and to end it a few minutes after tzeit. The custom is to wait about ten minutes past tzeit (SA 608:1; Peninei Halakha: Shabbat 3:1-2). Adding time shows that these days are beloved and very precious to us. Furthermore, by taking mundane moments and transforming them into sacred ones, we show the potential of the mundane. This transformation extends the day’s holiness to all the weekdays and elevates them.
It is a mitzva to verbally accept the holiness of the day. Women generally accept the sanctity of Yom Kippur when they recite the berakhot over candle lighting, when the day is invoked (see the next section). Men accept the sanctity of the day either upon reciting the berakha of She-heḥeyanu in the synagogue or by verbally accepting Yom Kippur (section 10 below).
One who concludes his pre-fast meal early may continue eating and drinking until he accepts upon himself the holiness of the day. If he was negligent and did not accept it upon himself before shki’a, all the Yom Kippur prohibitions nevertheless go into effect at shki’a (SA 608:1-3). One may accept the holiness of the day as early as plag ha-minḥa, which is about an hour and a quarter before shki’a (MB 608:14; Peninei Halakha: Shabbat 3:2 n. 2).