How is the date of Easter determined each year?
Andrea Mantegna, “Resurrection”, c.1457 (photo: Public Domain)
Catechism of the Catholic Church
1170 – At the Council of Nicaea in 325, all the Churches agreed that Easter, the Christian Passover, should be celebrated on the Sunday following the first full moon (14 Nisan) after the vernal equinox. Because of the different methods of calculating the 14th day of the month of Nisan, the date of Easter in the Western and Eastern Churches is not always the same. For this reason, the Churches are currently seeking an agreement in order once again to celebrate the day of the Lord’s Resurrection on a common date.
by Father Edward McNamara
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 ultimately depend on the date of 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 “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. Coincidence would have it that St. Teresa of Avila died on that very night of Oct 4-15.
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.