Why does Easter’s date vary from year to year?

Falling on March 23, Easter this year is almost as early as it possibly can be. The date of Easter is the Sunday following the first full moon after March 20.

But the full moon we’re talking about is not the astronomical full moon, but the Paschal full moon. The Paschal full moon’s possible dates were calculated by Christian astronomers in AD 325, when the Julian calendar was used and when the first day of spring was March 20. When Europe changed to the Gregorian calendar in the 16th century, the Church continued to use the Paschal full moon tables to determine Easter’s date.

The aim of dating Easter this way is ensure Easter always falls on a Sunday, the day of the week Jesus’ followers discovered he had risen, and to maintain for each Easter Sunday the same season of the year and the same relationship to the preceding astronomical full moon that occurred at the time of Jesus’ resurrection.

Before 325, Easter was simply celebrated on the first Sunday after the Jewish Passover or on or just after the first day of the Jewish Passover, which wasn’t necessarily a Sunday. Different Christian communities used one or the other, so Easter in one city might always be on a Sunday while in the next town it might fall on a Tuesday one year and Wednesday the next.

Because the first day of Passover is fixed (the 15th day of the Hebrew month of Nissan), the original relationship of Jesus’ crucifixion to Passover has been lost in the last 2,000 years. This year, Passover begins at sundown, April 19. Passover’s date is much more “mobile” than Easter’s because the Hebrew calendar is based on lunar cycles, not the solar cycle as the dating of Easter is.

The 40 days of Lent, preceding Easter, commemorate the 40 days Jesus spent in the wilderness in prayer and meditation just after he was baptized and just before beginning his ministry. Because every Sunday, not just Easter, is a celebration of the resurrection, the 40-day count excludes Sundays. Lent is intended to be a time for spiritual self-examination and devotion.