What causes this mismatch? There are two ways of answering this question. The first is that there’s a basic misalignment between the Christian and the Jewish festival calendars. Both holidays are supposed to fall on, or near, a full moon in the spring. Passover always begins on the 15th day of the Hebrew month of Nisan. Because the Hebrew months are pegged directly to the lunar cycle, the 15th day of Nisan is always a full moon.
For a time, early Christians used the Jewish calendar as a reference, celebrating Easter on the first Sunday after Nisan 15. But at the First Council of Nicaea in A.D. 325, the Church decided to set its own date for Easter, independent of the Jewish reckoning. Today most Christian communities celebrate Easter on the first Sunday after the first full moon after March 21. But sometimes this full moon isn’t the same as the Jewish one.
And here arises the second, deeper answer. 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. In fact, nothing is exactly the same length as a solar year, because not all solar years are the same length. This challenge ails not only both religious calendars, but also every human attempt at timekeeping on Earth.
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Allow Benjamin Dreyfus to explain. A professor of physics at George Mason University, he runs the Hebrew Calendar Facts page on Facebook.
“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. But that’s about 11 days too short: A solar year is about 365.2425 days.
If left unaddressed, 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.
Two thousand years ago, this decision was made on the fly, almost Groundhog Day–style. During the month of Adar (which directly precedes the Passover month of Nisan), the ancient rabbinical court would decide if it was springy enough outside for Passover. If spring seemed to be on track, Nisan could occur. But if it wasn’t warm enough outside yet, the rabbis would tack on another month of Adar. They called this leap month Adar II.
Around the third century of the Common Era, this observational system was replaced with a fixed calendar. The Hebrew calendar now adds a leap month seven years out of every 19. (Or, more exactly, Adar II is now added in the third, sixth, eighth, 11th, 14th, 17th, and 19th years of the cycle.)
“It works out so that over the course of 19 years, that comes out almost to the length of the solar years, ” Dreyfus said. “But it doesn’t work perfectly. The Jewish calendar drifts about one day later every 200 years, and so far there isn’t any mechanism to correct that.”