Here are dates of Easter and some other observances dependent on it:
2023 2024 2025 Ash Wednesday Feb 22 Feb 14 Mar 5 Palm Sunday Apr 2 Mar 24 Apr 13 Good Friday Apr 7 Mar 29 Apr 18 Easter Sunday Apr 9 Mar 31 Apr 20 Whit Sunday May 28 May 19 Jun 8
Ash Wednesday, the first day of Lent, is 46 days before Easter.
Palm Sunday, the first day of Holy Week, is 7 days before Easter.
Good Friday is 2 days before Easter.
Whit Sunday or Pentecost is 49 days after Easter.
Rule
The date of Easter is intended to be the Sunday after the full Moon that is on or after the day of the vernal equinox. But the equinox is equated with March 21 (whereas astronomically it may occur, in the Gregorian calendar, on March 19, 20, or 21). The date assumed for the full Moon, too, may not coincide exactly with the astronomical one. So Easter is in practice determined from a computus, or set of formulae. It can fall on any of the 35 days from March 22 (if full Moon occurs on March 21 and this is a Saturday) to April 25 (if full Moon occurs on March 20, thus next on April 18, and that is a Sunday). The nearest March 22 years are 1818 and 2285; the nearest April 25 years, 1943 and 2038.
Thus in 2023 the next full Moon on-or-after March 21 was April 6, a Thursday, so the next Sunday, April 9, was Easter.
Easter in turn determines many other “movable feasts” in the church year, such as Ash Wednesday (in the 7th week before) abd Whitsunday (7 weeks after).
History
The development of the rule was as follows. All Hebrew months begin just after new Moons, and Nisan, once the 1st month of the year (see Astronomical Companion, <H>CALENDARS<$>), began just after the new Moon nearest to the spring equinox. (Sometimes, as in 1986, Nisan actually begins with the next new Moon.) On the 14th of Nisan – the full Moon, or roughly so – is the feast of Passover or pesach (from which comes the adjective paschal). A paschal lamb is sacrificed, commemorating the time in Egypt when the angel of death “passed over” the houses marked with the blood of a lamb. At Passover in A.D. 33, Christ, having been sacrificed, was resurrected; and this in turn is what is commemorated by Easter.
In the Eastern church, this Paschal full Moon was Easter, whatever weekday it fell on. (The words east and Easter both go back to a root meaning “dawn.”) After a prolonged Paschal Controversy, at the Council of Nicaea in A.D. 325 the “quartodecimans” (from Latin quartus decimus, “fourteenth”) were overruled by the Western church’s formula, making Easter the Sunday after the Paschal full Moon. The Eastern churches still follow the Julian calendar, so that Easter falls on a different date.
There have been proposals (e.g. by the British parliament in 1928) to simplify the rule, even ignoring the Moon.
Pattern
Here is a graph, in which red dots are at the true days of full Moon, blue bars are at Sundays, abd year numbers are at Easter Sundays.
The distribution of the Easter Sundays over the calendar looks like a random scatter.
The irregularity arises because there are three factors: March 21, which is fixed; full Moons, which fall each year 10 or 11 or 12 days earlier in the month; Sundays, which become each year one day earlier in the month or, after a leap year, two.
If Easter was always just the first Sunday after March 21, or the first full Moon after March 21, our calendar would be considerably more regular – freed from its wobble, let’s say. One is tempted to think the Easter rule a nuisance like the clock-changing “daylight-shifting” rule!
There is much more on intricacies of the date of Easter in Jean Meeus, Mathematical Astronomy Morsels, 354-366. For instance, if calculated using the exact astronomical times of the equinox and of full Moon, the date of Easter would be different in 78 out of 1000 years. The nearest of these are 1981 and 2038.
Like Easter wandering with the moon
we seek through April into June…