Rosh Hashanah, 1 Tishrei

Rosh Hashanah opens on the first of Tishrei and, unusually among Hebrew festivals, this rule records two days rather than one even for communities that otherwise keep only a single day for other holidays, including in the Land of Israel itself. The reason traces to the same historical uncertainty that produced the Diaspora's extra day generally, the delay in news of a witnessed new moon reaching outlying communities, except that Rosh Hashanah falls on the first of its own month, the hardest possible day to confirm in advance, so even Jerusalem itself could not always be certain of the date until it had already begun. Rabbinic tradition therefore came to treat the two days as a single extended day, yoma arichtah, rather than as two separate days observed out of doubt.

The festival's textual basis is thinner than its later theological weight might suggest. The Torah names the first of the seventh month as a day of horn blasts and rest, Leviticus twenty three and Numbers twenty nine, without calling it a new year or a day of judgment. Those meanings, along with the shofar's later association with the Ten Days of Repentance leading to Yom Kippur, developed through rabbinic interpretation well after the biblical text itself was fixed.