The new day, in the Hebrew calendar, begins at some point between sunset and nightfall. Nightfall is defined as the time when is so dark that three medium size stars are visible in a clear sky. The length between sunset and nightfall is relative to the geographical location and the season of the year (for a great technical discussion about this issue see minhat cohen, by Rabbi Abraham haCohen Pimentel, Amsterdam 1668). In Israel, for example, this time is calculated at approximately 15 minutes. In New York, approximately 30 minutes. The window between sunset and nightfall is known as twilight or ben haShemashot.
Again, the new Hebrew day, any day, begins at some point between sunset and nightfall. This exact point cannot be established Halakhically. But we know that (i) the previous day does not end before nightfall and (ii) the next day does not begin before sunset.
The rabbis, therefore, established that Shabbat begins at sunset and ends at nightfall, i.e., the strictest possible consideration.
There is another element to bear in mind in order to understand the standard time Shabbat begins and Shabbat ends: our tradition (based on TB, Yoma 81b) is to add time to Shabbat, at the beginning and at the end of it. In other words, although Shabbat officially begins at sunset, we receive Shabbat before sunset. And although Shabbat ends at nightfall, we delay the end of Shabbat for a few minutes after nightfall.
In New York, for example, sunset is today at 4:57p.m.. The "conventional" custom is to receive Shabbat 18 minutes before sunset, i.e., 4:39 p.m. However, if we did not receive Shabbat at 4:39 p.m. we can still receive Shabbat a few minutes later (if, for instance we need to drive, cook, etc) as long as (i) we add a few minutes to Shabbat and (ii) we don't cross the sunset line.
(To be continued...)
Candle lighting in NYC: 4:39 p.m.
Shabbat ends in NYC: 5:37 p.m. *
* Calculating the end of Shabbat at 40 minutes after sunset. In many communities the end of Shabbat is determined at 50 minutes or more after sunset.
by Itzik Eshel