Passover (Pesach)

Passover must fall in spring. The Hebrew calendar is lunar. Those two requirements are incompatible without correction, and the correction is the calendar's central mechanism.

Twelve lunar months come to about 354 days, eleven short of the solar year. Left alone, the festivals would move backwards through the seasons at the rate the Islamic calendar does. The Hebrew calendar prevents this by intercalation: in seven years out of every nineteen, years 3, 6, 8, 11, 14, 17 and 19 of the cycle, a whole extra month is added, a second Adar, before Nisan. Nineteen solar years and 235 lunar months differ by about two hours, which is why the nineteen-year cycle is used by every lunisolar calendar that has ever worked.

The month begins from the molad, the calculated mean conjunction, whose value, 29 days, 12 hours and 793 parts, is accurate to within about half a second of the true mean and was established by Babylonian astronomers around 300 BCE.

Four postponement rules then adjust the start of the year, to prevent the Day of Atonement falling adjacent to a sabbath and to prevent certain awkward sequences, and these produce the six possible year lengths: 353, 354 or 355 days in ordinary years, and 383, 384 or 385 in leap years.

The system replaced an earlier practice of observation, in which witnesses reported the new moon to the court and an extra month was declared when the barley was not ripe. The transition was gradual, running from the first century to about the tenth; the traditional ascription of the fixed calendar to Hillel II in 359 is not well supported.

Almost everything about how Passover is kept varies along lines that are older than they look.

Diaspora Jews outside Israel keep the first and last days as two-day festivals; Jews in Israel keep one. The reason is calendrical archaeology. When the month's start was declared by a court in Jerusalem and communicated by signal fires and then messengers, distant communities could not be sure which day the court had chosen and kept both. The calculated calendar removed the uncertainty entirely, and the doubled days were retained anyway, as the custom of the ancestors. Reform Judaism dropped the second day; Orthodox and Conservative practice retains it.

The seder itself, with its four questions, four cups and ordered plate, is rabbinic. Its structure has been compared to that of a Greco-Roman symposium, with reclining, wine in courses and formal discussion, and the comparison is standard in the scholarly literature. The Haggadah grew over centuries and continues to grow.

The Samaritans, a community of a few hundred people, still perform a Passover sacrifice on Mount Gerizim, actual lambs, actual altar, annually. They are the only community that does. Their calendar is calculated separately and their Passover sometimes falls a month away from the Jewish one.

Karaite Jews, rejecting rabbinic authority, revert to observation of the new moon and of the ripening barley, which also produces dates that diverge.

The plagues, the departure and the historicity of the exodus are argued elsewhere; the festival's structure is what is being described here.