Easter Julian and Gregorian Calendar

Traditionally, the Eastern Orthodox and Byzantine Catholic churches use the Julian Calendar to calculate their feast days. Beginning in 1924 the Patriarchate of Constantinople made an adjustment to their liturgical year to bring the fixed cycle in conformity to the modern Gregorian Calendar. The Paschal cycle, however, continued to be calculated according to the Julian Calendar. This composite calendar is known as the Revised Julian Calendar. Constantinople's example was followed by the Church of Greece as well as a number of other autocephalous churches. Today, some churches continue to follow the Julian Calendar while others follow the Revised Julian Calendar.

(source: http://en.wikipedia.org/wiki/Byzantine_Rite#Calendar)

Julian Calender Easter Dates

2013 - Easter Sunday - May 5th
2014 - Easter Sunday - April 20th (same)
2015 - Easter Sunday - April 12th
2016 - Easter Sunday - May 1st
2017 - Easter Sunday - April 16th (same)
2018 - Easter Sunday - April 8th
2019 - Easter Sunday - April 28th
2020 - Easter Sunday - April 19th
2021 - Easter Sunday - May 2nd
2022 - Easter Sunday - April 24th
2023 - Easter Sunday - April 16th
2024 - Easter Sunday - May 5th

Gregorian Calender Easter Dates

2013 - Easter Sunday - March 31st
2014 - Easter Sunday - April 20th (same)
2015 - Easter Sunday - April 5th
2016 - Easter Sunday - March 27th
2017 - Easter Sunday - April 16th (same)
2018 - Easter Sunday - April 1st
2019 - Easter Sunday - April 21st
2020 - Easter Sunday - April 12th
2021 - Easter Sunday - April 4th
2022 - Easter Sunday - April 17th
2023 - Easter Sunday - April 9th
2024 - Easter Sunday - March 31st

The average length of a year in the Julian Calendar is 365.25 days (one additional day being added every four years). This is significantly different from the "real" length of the solar year. However, there is uncertainty among astronomers as to what the length of the solar year really is (see Simon Cassidy's Error in Statement of Tropical Year). The main competing values seem to be the "mean tropical year" of 365.2422 days ("mean solar days") and the "vernal equinox year" of 365.2424 days. The difference of the length of the Julian calendar year from the length of the real solar year is thus 0.0078 days (11.23 minutes) in the former case and 0.0076 days (10.94 minutes) in the latter case.

Inter GravissimasWhatever, this error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day. Thus as the centuries passed the Julian Calendar became increasingly inaccurate with respect to the seasons. This was especially troubling to the Roman Catholic Church because it affected the determination of the date of Easter, which, by the 16th Century, was well on the way to slipping into summer.

Pope Paul III recruited several astronomers, principally the Jesuit Christopher Clavius (1537-1612), to come up with a solution. They built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform. (The full text may be read in both the Latin original and a French translation by Rudolphe Audette, and in an English translation, recently done by Bill Spencer.)

In 1923 the Eastern Rite Churches adopted a modified form of the Gregorian Calendar. October 1, 1923, in the Julian Calendar became October 14, 1923, in the Eastern calendar. From this we see the celebration of Christmas on January 7th and New Year's on January 14th.

The date of Easter is determined by reference to modern lunar astronomy (in contrast to the more approximate lunar model of the Gregorian system). if you wish to see how these dates are calculated, the Astronomical Society of South Australia has an excellent page showing how it's done.