See also the calendar of festivals in Leviticus.
The Passover festival begins on an evening with a full moon, nowadays typically the first full moon after the spring equinox. Ultimately the complex motions of the moon around the earth and the earth around the sun lead to the variation in the date of festivals such as Passover and Easter.
A number of calendars were in use in first century Judea. An interesting example is that used at Qumran, preserved in the Qumran Scrolls[2]. The community there divided the year into 364 days, a number which is conveniently divisible into 4 seasons or 52 weeks of 7 days. Using this system, each festival would fall on the same day of the week every year. It also accommodated a lunar calendar of alternating 29 and 30 day months, requiring a only one 30 day month as correction every 3 years. This calendar differed from those used by the temple and the state, so they would each record events as occcuring on different days and dates.
References:
© David Billin 2005–2024
Occurrence[1]
March–April. Passover (see Numbers 9, Leviticus 23:5f)
April–May (e.g. 1 Kings 6:37)
May–June
June–July
July–August. Hottest weather
August–September
September–October. Start of the Civil Year
October–November (e.g. 1 Kings 6:38)
November–December
December–January
January–February. Coldest weather. End of the Autumn rains.
February–March. Purim (see Esther 9)