Passover (Pesach), 15 Nisan

Passover begins the fifteenth of Nisan on the Hebrew lunisolar calendar, and this rule records seven days for the Land of Israel and Reform communities against eight days for much of the traditional Diaspora, a split with a documented historical origin rather than an arbitrary difference. In antiquity, the start of each Hebrew month was fixed by witnessed testimony of the new moon before the Sanhedrin in Jerusalem, and word of the ruling was carried outward by messenger. Communities too far away to receive the news in time kept an extra day at the start of every festival as a hedge against uncertainty, a practice called yom tov sheni shel galuyot, the second festival day of the Diaspora.

The fixed calendar traditionally attributed to Hillel the Second in the fourth century removed the practical uncertainty the extra day had answered, since the calendar could now be calculated in advance anywhere. The second day nonetheless persisted in most traditional Diaspora communities as an inherited custom rather than a continuing necessity, while communities in the Land of Israel, and later the Reform movement on grounds of its own, kept only the original seven. Passover's textual basis is the Exodus narrative itself, read at the Seder alongside the eating of matzah and the avoidance of leaven for the festival's full length.