The Archbishop of Canterbury Justin Welby has recently announced that he has been in discussions with other Christian leaders to agree a fixed date for Easter. Pope Francis and various leaders of Eastern Churches have all expressed interest in finding a common date when all would celebrate Easter Day together.Easter was originally the Sunday following the Passover, the Jewish celebration of the liberation of the Israelites from slavery in Egypt, which occurs on the first full moon of the Hebrew year. But the start of the Hebrew year varied from year to year. Jewish months, 29 days long, mirror the phases of the moon, and so every three or four years an extra month is needed. After the destruction of the Temple in AD 70, there was no central Jewish authority to determine when to add that m