In Judaism, the day begins and ends at sunset. But defining sunset can be complicated. However, for many aspects of Jewish life, such as the observance of Shabbat/holidays, prayer times and even the day on which a baby boy has his brit milah (circumcision), knowing the exact moment when one day turns into the next is essential. There is, therefore, much discussion among the sages as to when day becomes twilight and twilight becomes night.
This time period is known in Hebrew as bein ha’shma’shoat, between the suns. As long as there is any trace of sunlight left, it is still twilight. According to the Talmud (Shabbat 34b): “From sunset on, as long as the eastern sky has a reddish glow, then, when the lower horizon is dark, but not the upper horizon, it is twilight; but when the upper horizon is as dark as the lower, it is night, so says Rabbi Yehudah. Rabbi Nechemiah says: [Twilight begins at sunset and lasts] as long as it takes a person to walk half a mil (.35 miles). Rabbi Yose said: Twilight is like the blink of an eye–the night comes and the day goes, and it is impossible to fix the exact time.”
Bein ha’shma’shoat is an interesting time in Jewish law and Jewish lore. For example, certain leniencies in Shabbat observance are allowed during bein ha’shma’shoat on Friday (e.g. one may directly ask a non-Jew to perform an act prohibited for Jews on Shabbat or one may separate certain tithes). In Jewish lore, according to Ethics of our Fathers (5:8), a number of mystical symbols such as the rainbow, manna from heaven and Moses’ miraculous staff, were created during bein ha’shma’shoat before the first Shabbat.