Zmanim · The moments of the day
What is bein hashmashot (twilight)?
Bein hashmashot is the twilight between the shkiah (sunset) and tzet hakochavim (nightfall). Because it cannot be known whether it is still day or already night, the tradition treats it strictly on both sides.
The time that belongs to neither day nor night
The phrase בין השמשות, bein hashmashot (between the suns), names the interval that begins at the shkiah, the sunset, and lasts until tzet hakochavim, the appearance of the stars. In this window the tradition does not claim to know whether it is still day or already night — and so, rather than choose, it holds both possibilities at once.
Where this is taught
The Talmud, in tractate Shabbat (34b), treats this twilight as genuinely doubtful, and therefore counsels stringency on both sides: what is forbidden by day and what is forbidden by night are both kept during it.
What to do
This is why Shabbat is welcomed a little before the twilight and taken leave of a little after it: we add from the ordinary week onto the holy day at both ends. The length of bein hashmashot depends on your place and season, so lean on the times reckoned for your own city.
Source: Talmud, tractate Shabbat 34b.
To hold both edges of this twilight with confidence, let Kimá (the Jewish clock: your city's zmanim and candle times) mark exactly when your day is doubtful and when it is sure.
Open Kimá