Independence Day in the United States falls on July 4, year after year. New Year’s Day is always on the first of January. Christmas occurs on December 25, unless you are part of the Orthodox Church or live in Foula. Then we have Easter. There seems to be no rhyme or reason for its appearance on the calendar. Why is Easter never on the same date?

The answer isn’t all that cut and dry.

Easter is a Christian holy day honoring the resurrection of Jesus. Jesus was crucified at the time of the Jewish Passover feast, so Easter is closely linked with that date. So when is Passover? It is determined by the lunar calendar. The date for Passover corresponds with the first full moon after the vernal equinox (first day of spring). The vernal equinox can occur as early as March 19 or as late as March 21.

The Council of Nicea in 325 established Easter as the first Sunday after the first full moon after the vernal equinox. From that point forward, the Easter date depended on the ecclesiastical approximation of March 21 for the vernal equinox. Easter is delayed one week if the full moon is on Sunday, which decreases the chances of it falling on the same day as the Jewish Passover. The council’s ruling is contrary to the Quartodecimans, a group of Christians who celebrated Easter on the day of the full moon, 14 days into the month. This means that they earliest date for Easter could be March 22, and the latest could be April 25.

Complicated enough yet? Well, this all assumes you are using the Gregorian calendar. Some churches calculate the date of Easter using the Julian calendar.  The whole reason the Gregorian calendar was created was to address inaccuracies in the calculation of the vernal equinox, so if you base your calculations of Easter on that system, as do the Orthodox churches, Easter drifts further and further away from Passover.

According to the Julian calendar, Easter also falls on a Sunday from March 22 to April 25, but that equates in the Gregorian calendar to being from April 3 to May 10.

In 1818, the full moon occurred on March 21 (the equinox). Therefore, the following day, March 22, was Easter. It will not fall as early again until 2285. In 2008 Easter came almost as early — March 23. It had last occurred that early in 1913. The next time that will happen will be 2160.

The latest Easter can occur — April 25 — happened last in 1943. That year, the full moon fell on the day before the equinox. The next time it will happen that late will be in 2038. In 2011, Easter occurred at the next-latest date of April 24. That will not happen again until 2095.