Why does the Jewish calendar add a leap month, and why does each day begin at sunset?
Twelve Moon months fall about 11 days short of a solar year, so without a fix Passover would wander through the seasons.
▶ Start the storyThe Jewish calendar is built on the Moon but kept in step with the seasons, and the leap month is the trick that makes both work. Its months follow the Moon and last 29 or 30 days. Twelve of them make just 354 days, which is shorter than the solar year, so an extra month is added every 2 or 3 years. Today leap months are added in 7 out of every 19 years, a pattern called the Metonic cycle. The calendar is used for Jewish religious observance and fixes the dates of holidays and the scheduling of public Torah readings.
The reason is the festivals' seasons. The insertion of the leap month is based on the requirement that Passover fall at the same time of year as the spring barley harvest, so without leap months Passover would drift round the seasons. Maimonides explained that the solar year exceeds the lunar year by approximately 11 days.
Step 1: 12 moons
Just 354 days
Step 2: The solar year runs longer
By about 11 days (Maimonides)
Step 3: Passover would drift
It must stay at the spring barley harvest
Step 4: Add a month
Every 2 or 3 years
Days work differently from ours. Following the rabbinic reading of Genesis 1:5, "there was evening and there was morning", a day runs from sunset to the next sunset. That is why Shabbat observance begins before sundown on Friday, and ends after nightfall on Saturday, to be sure it is not violated whichever moment counts as the change of day.
How were months fixed? Once, each month began when witnesses reported the new crescent to the Sanhedrin. Between the years 70 and 1178 that observation was gradually replaced by mathematical rules, and tradition credits Hillel II with introducing the fixed leap cycle.
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Recap
Twelve moons are 354 days, so a thirteenth month is slipped in 7 times every 19 years.
💡 A trick to remember it · Seven in nineteen: the Moon's calendar buys a month to catch up with the Sun.
Surprising fact · Leap months fall in 7 of every 19 years, and the month names came from Babylon.
Sources (1)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.