It's the last remnant of the Babylonian lunar calendar in the west. It was a lunisolar year- that is, one in which the months are based on the moon. But because the lunations don't go evenly into the year (there's about 11 days left over), leap- months are added at intervals of 2 or 3 years to bring it back in step. The Jews adopted this system during their exile, and as it was used to calculate the Passover, it is used for Easter.

The early Christian Church adopted it for Easter- I'm not sure why, as all saint's days, Christmas etc. are fixed to the Roman (Julian) calendar- but changed the calculation when one pope realised that Christians and Jews were celebrating on the same day- you can't have that!

The change had unforeseen consequences when Rome sent missionaries to England. Parts of England had been christianised by Irish missionaries, but their church had been isolated from Roman influence for over a hundred years, and they hadn't heard about the change. So Irish converted Christians celebrated Easter at different times from Roman converts. Because of the huge range of dates, one set could have just finished their pancakes when the other lot were munching chocolate eggs. This combined with the fact that, when the British Church had last been seen just after the fall of Roman Britain, it had been much influenced by the Pelagian heresy (don't bother to ask). The Romans took the Easter difference as evidence of heresy, and set out to remove the Celts. The Synod of Whitby (which wasn't at Whitby because Whitby hadn't been invented then) and a bit of realpolitik saw the tolerant Celts banished back to Ireland in disgrace, leaving behind only their script and the tradition of manuscript illumination .

Now wouldn't it be a lot more sensible if we could settle on a civil Easter, around the equinox or just after, and allow the Christians to celebrate their religious feast at the correct but inconvenient date, just as Moslems live with a moving Ramadan, Hindus with Diwali and so on?