Today is Ash Wednesday, the first of day of Lent, and 46 days before Easter, the original movable feast. Why do some Christian holidays move around while others, like Christmas, always fall on the same day?
Because the church said so in A.D. 325. The date of Easter is determined according to the lunar calendar, while the date of Christmas is fixed on the solar calendar. Before 325, there was no official celebration of the birth of Christ, and Easter was celebrated by some Christians on Passover (a lunar holiday) and by others the following Sunday. The rationale: Christ's last supper took place on or around Passover, he was crucified on a Friday, and the festival of Easter celebrates his resurrection two days later.
In 325, church officials at the First Council of Nicaea formalized the date of Easter in an effort to get everyone to celebrate on the same day (and also, possibly, to dissociate it from the Jewish Passover feast). From then on, the holiday was celebrated on the Sunday following the first full moon after March 21, the start of spring.
At the same time, the council inaugurated Christmas by making Dec. 25 the Feast of the Nativity. Because Christmas was not directly related to a lunar holiday, and because it had never been celebrated before—the date of Christ's birth is not mentioned in the Bible, and questions about it had been settled by a proclamation from the pope just five years earlier—the council was able to establish an unambiguous date for the celebration.
But Easter remained tough to pin down. The Julian calendar in use at the time relied on the leap year to keep seasons from drifting with respect to dates, but the correction only worked so well. By the mid-16th century, the vernal equinox was occurring more than a week before March 21.
In 1582, Pope Gregory XIII solved this problem by issuing a papal bull that eliminated 10 days from the calendar that year. Oct. 4, 1582, was followed immediately by Oct. 15, 1582. The start of the following spring occurred on March 21, and Easter was restored to its proper season. To avoid future problems, Pope Gregory abolished leap years for every year that is evenly divisible by 100, except those that are evenly divisible by 400. (So, the year 2000 had an extra day, but 1900 didn't and 2100 won't, either.)
In theory, Easter falls on the Sunday after the first full moon following the vernal equinox. In practice, the church's methods are a bit imprecise, in part because the vernal equinox doesn't always fall on March 21 and in part because the church uses traditional tables (rather than modern astronomy) to determine the dates of full moons.
Eastern Orthodox churches never adopted the Gregorian calendar, so their Easter remains on the Julian calendar. This year it will fall on May 1, five weeks after the Western Easter.