The inherent sanctity of Shabbat is present only from the beginning of the seventh day, when Shabbat starts. However, the Torah commands that we extend the sanctity of Shabbat into the mundane week. That is, we are meant to accept the holiness of Shabbat upon ourselves a bit before the start of the seventh day. The same applies to the end of Shabbat: it ends at the end of the seventh day, but we are commanded to extend this holiness a little longer after Shabbat (SA 261:2; Bi’ur Halakha states that most authorities consider this extension to be a Torah law).
This extension of Shabbat, called tosefet Shabbat, demonstrates that Shabbat is very dear to us. We go out to greet it before its arrival, and we prolong its stay by accompanying it upon departure. It is like an honored guest whom we go out to greet and whom we escort when it is time to take leave.
We have already learned that the status of the time between shki’a and tzeit is uncertain, and thus one must be stringent then. Therefore, in order to fulfill the mitzva of tosefet Shabbat we need to accept Shabbat a bit before shki’a. Women customarily accept Shabbat when they light the Shabbat candles. In Jerusalem the custom is to light candles forty minutes before shki’a, in Haifa thirty minutes before, and in Tel Aviv and most other Israeli cities twenty minutes before. Men who recite the Minĥa prayers close to shki’a are accustomed to accept Shabbat later, but they too must be careful to accept Shabbat a few minutes before shki’a in order to add to the holy from the mundane.
One who wishes to accept Shabbat even earlier is commendable, as long as it is within a seasonal hour and a quarter before shki’a. According to many poskim, one may not accept Shabbat earlier than this (SA 263:4; MB ad loc. 18).
Shabbat concludes with the appearance of three medium-sized stars, but the Sages were worried that people might make a mistake and think that three large stars were medium-sized. Therefore, to be on the safe side, they said that one should wait until the appearance of three small stars grouped together (SA 293:2). Nowadays we do not need to look at the stars. Rather we can rely on clocks and calendars, where the times listed for the end of Shabbat already incorporate tosefet Shabbat.
This mitzva teaches us that there is a connection between the weekdays and Shabbat, which is why we can add from the mundane to the sacred. We can also see, based on this, the inner striving of the mundane to be connected to the sacred.
. As we saw in n. 1 above, the longest time between shki’a and tzeit in Eretz Yisrael is just under 30 minutes (at the height of the summer in low-lying areas). Calendars nowadays generally record the end of Shabbat as 35 minutes after shki’a, so that there is always tosefet Shabbat.