The Paschal Feast, Passover, and other Liturgical Celebrations

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The Jewish calendar

Around 1500-1600 years ago, the Jewish calendar rules, formerly a secret science, were published, and from that time the calendar was computed rather than observed. The calculation has a smaller error than the Julian calendar, but a larger error than the Gregorian calendar, causing the Jewish year to advance on the equinox by 1 day every 216 years. At some time in the future, a Pan-Judaic Synod may need to adjust the calendar, otherwise Pesach will eventually start occurring in the summer. The intention of the calendar rules is to make the vernal equinox fall in the first half of Nisan, as commanded in Exodus 12:18. There's a bit more to it than that, but that's the general intent.

Here's an article:

The Julian and Gregorian Calendars

The old Julian Calendar (adopted in 45 B.C.) had an error of about 1 day in 133 years. The Gregorian calendar has an error of one day in 3300 years. The date of the Paschal celebration was set in 325 to be calculated based on an ecclesiastical calculation of a theoretical month (Paschal month) rather than actual lunar month. The Paschal month will contain March 21st in its first half. March 21st was the date of the vernal equinox in 325, and by the rules decided at the Council of Nicaea, from then on neither the actual date of the vernal equinox nor the actual lunar month were to play any further role in the calculation.

When Gregory corrected the Julian calendar in 1582, he also determined that the new calculation of the date of March 21st would be used to determine the date of the Paschal feast, but otherwise retained the Nicaean calculation of the Paschal lunar month. The Churches of the East objected to this change, and even if they use the Gregorian calendar to determine other dates (events which occurred or were celebrated on calendar dates rather than on dates relative to the Paschal cycle) they prefer to use the unmodified Nicaean calculation, including the error in the calculation of the length of the year and thus which March 21st to use, the "old" one or the "new" one, when calculating the date of the Paschal feast. This will cause the Eastern calculation of the observance of the Resurrection to move out into the summer, almost twice as fast as the current Jewish calendar is moving Pesach into the summer.

Prior to 1923, the Julian calendar was still in use in some countries. The Gregorian calendar was first adopted by Spain and her colonies (including Florida) the day following Oct 4th, 1582, which was reckoned as Oct 15th. Gradually, other nations adopted the Gregorian calendar: the Catholic German states in 1583; the Protestant German states in 1699; Great Britain and her colonies in September 1752; Sweden in 1753; Alaska upon purchase in 1867; Japan in 1873; China in 1912; the Soviet Union in 1918; and Greece in 1923. In Switzerland, both calendars were in use from 1583 until the country fully adopted the Gregorian calendar in 1812.

Lots of info on church date calculations:

In the current thinking in the Eastern Church, following the directives of a universal council is more important than the tradition of celebrating the Paschal feast in the spring. I would agree with them about councils with respect to "first things" (important theological issues), but in my opinion the tradition of the Paschal feast being in the Spring allows for an interpretation of the Nicaean decision which would use the secular calculation of March 21st (currently the Gregorian calendar) in the Nicaean formula.

The following links will display OpenVMS DCL procedures for date calculations in a new window: