Quartodecimanism (from the Vulgate Latin quarta decima in Leviticus 23:5,[1] meaning fourteenth) is the practice of celebrating Easter on the 14th of Nisan at the same time as the Jewish Passover.
[7] The Roman missionaries coming to Britain in the time of Pope Gregory I (590–604) found the British Christians adhering to a different system of Easter computation from that used in the Mediterranean basin.
This system, on the evidence of Bede, fixed Easter to the Sunday falling in the seven-day period from the 14th to the 20th of its lunar month, according to an 84-year cycle.
[10] The 84-year cycle, the lunar limits, and an equinox of March 25 also receive support from McCarthy's analysis of Padua, Biblioteca Antoniana, MS I.27.
[14] After the Gregorian reform of the calendar by promulgation in 1582, the Catholic Church continued to follow the same method for computing the date of Easter but the resulting date differed from that computed using the Julian Calendar due to the difference in time regarding when the vernal equinox was deemed to occur and when the relevant full moon fell.