By Denise Winterman
BBC News Magazine
So it's Pancake Day already? Shrove Tuesday heralds the start of an unusually early Lent and Easter. Why does the most important season in the Christian church hop around the calendar more than an Easter bunny?
The date of Easter is one of those subjects which, though passing unquestioned 99% of the time, is loaded with significance, argument and history.
The entire Roman province of Asia was excommunicated for several centuries for heresy over the date and Christians in England were split over it for decades. But the rules for calculating the date of Easter Sunday are complex and remain controversial.
In the early church, Easter was celebrated on different days in different countries. The date was bitterly disputed until the council of Nicaea in AD 325 decreed the festival would be observed on the same date.
While the council may have desired consistency, simplicity took a back seat.
The day chosen was the Sunday following the first full moon after the spring equinox. The full moon being a notional "Paschal" full moon which occurs in either a 29 or 30-day cycle, not the astronomic moon we can all see in the heavens.
The theological significance of this date was that it was a day of maximum light -12 hours of daylight, followed by 12 hours of full moonlight. It means that Easter can be celebrated on a Sunday between March 22 and April 25. So although this year's date of 27 March is early for Easter, it is by no means the earliest possible date. It does, however, coincide in the UK with the clocks going forward.
Accurately calculating Easter was important to early Christians as they believed celebrating it too soon or too late was sacrilegious. But the calculation tables used have often been a matter of major controversy and various system have been adopted and rejigged over the centuries.
After the ferocious early schisms and excommunications, the Gregorian calendar was adopted in Britain and Ireland in 1752 and Easter has been celebrated on the same day in the Western part of the Christian world ever since.
The Eastern churches, however, did not adopt the same calendar and still celebrate Easter at a different time. Occasionally the dates coincide, for example, in 1865 and then in 1963.
Discussions still continue over the advantages of a fixed Easter. In 1928 the House of Commons agreed to a bill fixing the date as the first Sunday after the second Saturday in April. It was never implemented.
In 1990 the Vatican approved a proposal for a fixed date of Easter, coming out in favour of it as long ago as 1963. The proposal is subject to agreement with all other Christian churches and with governments. So far agreement has not been reached so Easter still drifts from year to year.