The Java Script Easter Sunday/Jewish Passover calculator is based on mathematical algorithms similar to those first devised by the German mathematician Carl Friedrich Gauss (1777-1855).
The Gaussian algorithm for calculating the first day of the Jewish Passover feast (15 Nisan) is equivalent with the method currently employed in the rabbinic (or cyclic) Jewish calendar, which according to tradition was introduced by Hillel the Patriarch (Hillel II) in AD 358/359 (AM 4119). Stern, 2001) that up to the 10th century AD several versions of the Jewish calendar existed simultaneously and textual evidence suggests that some of the underlying parameters and postponement rules currently employed in the rabbinic Jewish calendar were probably not introduced until the mid-10th century AD.
when the Dominical Letter is E) Easter Sunday is observed a week later on 13 April (instead of 6 April).
This happens four times in each 532-year cycle and since the introduction of the Dionysian Easter reckoning this has occurred in 570, 665, 760, 1007, 1102, 1197, 1292, 1539, 1634 (23 April New Style), 1729 (24 April NS) and 1824 (25 April NS).
The next cases (up to 2500) will be in 2071 (26 April NS), 2166 (27 April NS), 2261 (28 April NS) and 2356 (29 April NS).
Of course there will also be differences with the Easter dates computed from the earlier Paschal tables (based on 16- and 84-year cycles) that were previously adopted in the Western Roman Empire.
Note: A slightly different Easter reckoning is followed to this day by some Eastern Orthodox Churches, notably the Armenian Orthodox Church, the Syrian Orthodox Church (known as the Jacobites) and the Church of the East (known as the Nestorians).
The latter are also known as the Non-Chalcedonian churches and, as the Armenian Orthodox Church, are located near the eastern border of the former Byzantine Empire.
These churches place the Paschal Full Moon on 6 April (instead of 5 April) when the Golden Number is 1 and when this happens to fall on a Sunday (i.e.
The following Java Script calculator determines the dates of Julian and Gregorian Easter Sunday and the date and weekday of the first day of the Jewish Passover feast (15 Nisan) for a given year.
Also given are the dates of the Paschal Full Moon and miscellaneous computistical parameters such as the Dominical Letter, the Golden Number, the Epact, and the martyrology letters according to the Julian and the Gregorian calendrical reckoning.
The first day of the Jewish Passover feast is given according to the Julian calendar up to 1582 (AM 5342/43) and according to the Gregorian calendar from 1583 (AM 5343/44) onwards.
Note that the Jewish Paschal Full Moon always falls on 14 Nisan.