There are essentially three major categories of Jewish holidays, celebrations, and commemorations found in the Jewish calendar. These are biblical holidays, rabbinic holidays, and post-rabbinic celebrations. These categories indicate the historical period during which these holidays came to be established events in the Jewish calendar.
All Jewish holidays begin the evening before the date specified on most calendars. This is because a Jewish "day" begins and ends at sunset, rather than at midnight. If you read the story of creation in Genesis Ch. 1, you will notice that it says, "And there was evening, and there was morning, one day." From this, we infer that a day begins with evening, that is, sunset. Holidays end at nightfall of the date specified on most calendars; that is, at the time when it becomes dark out, about an hour after sunset.
Chag Sameach!
