Why (in some years) Easter Is Celebrated on Different Dates
In determining the day to celebrate Easter, early Christians faced a dilemma. It was known that Christ was crucified after Passover and therefore the date for Easter should fall after Passover. The date for Passover is the fourteenth day of Nisan (the first month of the Jewish ecclesiastical year, about the time of the vernal equinox), a fixed date in the Jewish lunar calendar. This date, translated to the old Julian or solar calendar that Christians used, became a floating date that fell any time in a week, and therefore made the date and day for Easter change yearly. To add to the confusion, early Christians felt that Easter should always fall on a Sunday. This was resolved at the Council at Nicaea in 325 A.D. when the date for Easter was set as the Sunday that fell after both the fourteenth of Nisan and the vernal equinox.
Further controversy in the date of Easter began in 1582 A.D. with the introduction of the Gregorian calendar. This calendar was not accepted by all countries, and even today, there are many churches that still use the old Julian calendar.
Currently, churches on the Gregorian calendar calculate Easter as the first Sunday after the full moon that comes on or after the vernal equinox (March 21). This means that Easter can fall within a 35-day period between March 22 and April 25, inclusive. Churches that still use the old Julian calendar occasionally have Easter on the same Sunday as those on the Gregorian calendar, but through the different methods of calculation may celebrate Easter anywhere from one to five weeks later. This is due to a combination of factors including the thirteen-day lag behind the Gregorian calendar and the tradition that Easter must necessarily follow the Jewish Passover but must never precede or coincide with it.