Why does Easter move?

One day it came up in conversation: why does Easter move around the calendar? Christmas doesn’t do that. Our birthdays don’t do that. What’s so special about Easter? We started asking around. Even the people who normally have answers for everything gave bluffy politician-type answers.
‘It’s based on the old Hebrew calendar, and the cycles don’t quite fit,’ said one guy, who edits encyclopedias for a living.
‘But it shifts, like, a whole month from one year to another.’
‘They really don’t fit.’
Google, as ever, came to our rescue. It was all decided, it seems, back in 325AD at the first Council of Nicaea, convened by the Emperor Constantine. Different countries were celebrating Easter on different days, and the emperor, head of the Christian church, wanted a simple solution to the problem. He didn’t get one. The Council decided that Easter should be held on the first Sunday after the first full moon after the spring equinox. This would give the holiest of days maximum light, day and night, as there would be around twelve hours of daylight, and around twelve hours of moonlight. Unfortunately it also meant that Easter could fall anywhere between March 22nd and April 25th.
In 1990, after 1,665 years of confusion over this frankly bizarre compromise (based on the Hebrew celebration of Passover) the Vatican approved the idea of a fixed-date Easter. JP2, it seems, was tired of arguments about different calendars meaning different Easters in Western and Eastern Europe, and disputes over the difference between an ecclesiastical full moon and an astronomical full moon. This approval has never been ratified. Frankly, enjoying all the unholy disruption it causes, we hope it never is. AL

A Moveable Feast: why does Easter shift around?