How the Date for Easter Is Determined Each Year
It harkens back to the holiday's pagan origins.
Some holidays, like Independence Day and Christmas, have set dates for celebration. Other holidays, like Thanksgiving on the fourth Thursday in November, have a consistent calendar for when we celebrate them. Easter, on the other hand, seems to change from year to year—sometimes, we celebrate in March and other times we celebrate it in April. So, how is the date for Easter determined each year?
Let's start by talking about the history of the holiday. Easter, a Christian holiday, has pagan roots that coincide with the concept of rebirth: a celebration of spring when the world is in bloom again. Pagans would often base their festivals on a cosmic event such as the shape of the moon or the constellations in the visible sky. So, while Easter is a culmination of several holy days that observe the trial, death, and resurrection of Jesus Christ, the date itself for Easter is tied to the moon. Easter is traditionally celebrated on the first Sunday following the full Moon—officially called "paschal full moon"—that lands on or just after the spring equinox.
This year's spring equinox will take place on March 19. According to The Farmer's Almanac, the word equinox refers to the length of time for day and for night being equal. The equal balance of daylight and nighttime is virtually the same all over the world on this date. Northern Hemisphere continents, like North America, begin to receive longer daylight because the planet tilts a bit more toward the sun. It gets warmer for us and flowers start to bloom.
Things get more complicated when that happens. When Pope Gregory XIII first set the rules for Easter in the 1500s, he also determined that the Church would always observe the date of the spring equinox on March 21, if it happened to occur too early in the calendar. Easter would also need to always fall between March 22 and April 25. So, if the spring equinox and the paschal full moon occurred on the same day and were before March 21, then that full moon essentially did not "count" toward the Easter date calculations. The next full moon was then used to calculate the Sunday observance for Easter.
These rules were applied last year in 2019 when the full moon and equinox occurred on March 20, and will kick in again in 2038 when the spring equinox will take place on March 20 and the paschal full moon will be on the following Sunday, in which the rules dictate that Easter will be observed on April 25 and not on March 28 as we would otherwise expect. It gets even more confusing when the "ecclesiastical full moon" rules are used for choosing the date for Easter. The Church created tables for this full moon that does not always coincide with the full moon observable in the sky.