The Date of Easter

Why is Easter on a different Sunday each year? It is the most important day in the Church’s calendar (more important than Christmas), yet the day we commemorate our Lord’s Resurrection is not fixed to a particular date like our Lord’s Birth is. Well, Easter follows a lunar, rather than a solar, calendar and is celebrated on the Sunday that follows the first full moon after March 21, the vernal (spring) equinox. Therefore, Easter cannot fall earlier than March 22 or later than April 25.

All the other movable celebrations in the Church calendar, such as Ash Wednesday, Pentecost, and Corpus Christi, ultimately depend on the date of Easter. Depending on when the date of Easter for the year is determined to fall, those other feasts move around a little bit in response to Easter.

Most of the Eastern Churches follow the same basic principles but often celebrate Easter on a date different from Catholics and other Western Christians because they continue to follow the calendar of Julius Caesar without the corrections incorporated by Pope Gregory XIII in 1582. Julius Caesar’s calendar calculated the year as 365 days and 6 hours, and thus was about 11 minutes and 9 seconds more than the sun’s actual course. Although tiny, this excess puts the calendar off by a day, more or less, every 128 years. Thus, the Council of Nicaea already found it necessary to regress the date of the spring equinox to March 21 instead of the original date of March 25. By the time of Pope Gregory XIII, the difference had grown so much that the spring equinox occurred on March 11.

In 1581, with the bull (decree) “Inter Gravissimas,” Pope Gregory promulgated a widespread reform which, among other things, re-established the spring equinox on March 21 by eliminating 10 days from October 1582. The error of Julius Caesar’s calendar was corrected by deciding that the turn of the century — always a leap year in the Julian calendar — would be so only when the year could be divided by 400, that is, 1600, 2000, 2400, 2800, etc., whereas there would be no leap year in the others.

Most Catholic countries, and even some Protestant ones, accepted the reform almost immediately. Some countries, such as England, held off accepting the papal reform until 1752, while Russia did not adopt it until after the Communist takeover in 1918.

The calculation is still not perfect, as there is still a difference of 24 seconds between the legal and the solar calendar. However, 3,500 years will have to pass before another day is added.