[Up Yours Marxists]

I always wondered why we flop around so much for Easter. We know when Jesus died and rose from the dead. Just make it that day and call it kosher.

[Paladin2]

Calendar based on phases of the moon, like Jewish and Muslim calendars, except the Romans got things largely on a solar basis.

[Lonesome in Massachussets]

No one know precisely when Jesus rose from the dead, or when he died. The date of Easter was set to make it fall after Passover. It was agreed that Christ died on a Friday, after the Seder on Holy/Maundy Thursday. Passover starts on the 15th of Nissan, the Hebrew lunar month which starts on the first new moon after the vernal equinox.

By this reasoning Easter is on the first Sunday on or after the first ecclesiastic full moon after the ecclesiastic vernal equinox. Gregorian calendar reform was motivated by the need to make the ecclesiastic vernal equinox, fixed on March 21 align with the astronomically determined date, and to occur on the same date in all places in the world. The Gregorian Calendar has a side kick lunar calendar, accurate to about one day every 8000 years, for calculating lunar cycles. An ecclesiastic full moon occurs when the “age of the moon” (days into the lunar cycle) equals 15.

https://en.wikipedia.org/wiki/Computus

[Lonesome in Massachussets]

This code is only valid for years in the Gregorian calendar.

Java - Anonymous (also called Meeus/Jones/Butcher) Gregorian algorithm

public static Date getEasterDate(int year) {
int a = year % 19;
int b = year / 100;
int c = year % 100;
int d = b / 4;
int e = b % 4;
int f = (b + 8) / 25;
int g = (b - f + 1) / 3;
int h = (19 * a + b - d - g + 15) % 30;
int i = c / 4;
int k = c % 4;
int l = (32 + 2 * e + 2 * i - h - k) % 7;
int m = (a + 11 * h + 22 * l) / 451;
int n = (h + l - 7 * m + 114) / 31;
int p = (h + l - 7 * m + 114) % 31;
Calendar calendar = GregorianCalendar.getInstance();
calendar.clear();
calendar.set(year, n - 1, p + 1);
return calendar.getTime();