The first is that there’s a basic misalignment between the Christian and the Jewish festival calendars.
For a time, early Christians used the Jewish calendar as a reference, celebrating Easter on the first Sunday after Nisan 15.
The lunar mismatch occurs because both calendars must grapple with the same underlying problem: A lunar year is not the same length as a full solar year.
“The Hebrew calendar uses lunar months, and they’re about 29 or 30 days each. If you have 12 of those months, it adds up to 354 days,” Dreyfus told me.
This would quickly cause the Hebrew calendar to drift out of sync with the solar calendar, violating the biblical commandment to celebrate Passover during the spring.
The Hebrew calendar resolves this tension by periodically adding an extra month to the calendar.
The Hebrew calendar now adds a leap month seven years out of every 19.
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