Why does Easter fall on a different day every year?
Where most holidays, such as Christmas and New Year’s Day, always fall on the same day of the year, Easter is different. How did that happen?
Easter and Passover
Easter is usually celebrated in the Jewish month of Nisan, the same week as the Jewish Passover (which begins on Nisan 15 and lasts one week). After all, the crucifixion of Jesus took place during Passover. Judaism has a hybrid solar-lunar calendar, i.e. a lunar calendar that is adjusted to the solar calendar every year.
Jewish calendar cross between solar and lunar calendar
That is, each Hebrew month has 29 or 30 days, which is the time between two new moons. The exact start date depends on the appearance of the new moon. The orbital period of the Earth around the Sun is 365.25 years. This is about 11 days more than the number of days in 12 Hebrew lunar months. For that reason, every few years a thirteenth intercalary month behind the month of Adar, Adar II, is added to the Jewish calendar. So being able to count well is very useful if you are a rabbi.
Bishops are stepping it up a notch
To show that Christians really differ from the Jews, the Christian bishops decided at the Council of Nicaea (now Nice) to do things differently. Of course the Christians did not use the lunar calendar, but the Easter date does show its influence. Easter always falls on the first Sunday after the first full moon after the vernal equinox (March 21). So if the full moon is exactly on March 21, Easter will fall a month later, at the next full moon.
Are you still following? Good, because it gets even more complicated. Orthodox Christians still follow the Julian calendar instead of the Gregorian calendar. Therefore, the date when Russian Orthodox Christians observe Easter differs from the date when Western Christians do. Calculating the Easter date is therefore not an egg, but fortunately it is doable.
