Setting the date for Easter is an astronomical problem. Easter Day falls on the first Sunday, after the first full Moon after the vernal (spring) equinox. Because of this complicated rule, the date changes from one year to the next. This year it will be on April 12th, but last year, it was March 23rd - almost three weeks earlier! To establish the date in advance, we need to calculate the positions of both the Sun and the Moon. The history of how this came about is an interesting one.
Easter is a celebration of the Resurrection of Christ. We know that his Crucifixion happened during the Jewish Passover festival which takes place in the spring. That is why Easter is in the spring.
In the early days, most Christians lived in the Middle East. For them, choosing the appropriate day to celebrate was not complicated. They merely relied on their local rabbis to announce the date when Passover began and then observed Easter a few days later.
This practice changed in the fourth century. By that time, Christianity was the official religion of the Roman Empire and the emperor, Constantine, wanted all Christians to celebrate the Resurrection on the same day. Also he did not want rabbis to determine when that day should be! To settle this and other important issues, he convened a conference in 325 AD. Our rule for setting the date of Easter is a result of his conference.
There are two facts in the Bible that relate to the date of Easter. The Last Supper was held on the first day of Passover and the Resurrection was the day after the Jewish Sabbath.
Since the Jewish Sabbath is on Saturday, it is straightforward to conclude that Easter should be celebrated on a Sunday.
Selecting a date for the beginning of Passover, years in advance, is much more complicated. The Roman calendar (also our calendar) is based on the Sun; it is synchronized with the seasons. However, the dates for Jewish feasts are set according to a lunar calendar. A twelve-month lunar year has only 354 days: eleven days shorter than our year. In order to keep the Jewish lunar calendar in step with the seasons, they insert an extra month every few years. Thus some years have twelve months and others have thirteen months! In any case, we know that Passover begins on the 15th day of Nisan, which is usually the day of the first full Moon in the spring. So in our system, the day of the first full Moon after the vernal equinox is equivalent.
Thus the rule: the first Sunday, after the first full Moon after the vernal equinox. For setting the date of Easter, the vernal equinox is deemed to be on March 21st. Actually, because of our leap years, it often falls on March 20th, but in 325 AD it was on March 21st.
But that is not the end of the story! Our story takes us back to 46 BC in Rome. That is when the solar calendar was first introduced to the Roman Empire by Julius Caesar, who had learned about its advantages from Egyptian astronomers. Prior to that, the Romans had followed a lunar calendar.
Caesar's new calendar, the Julian calendar, was based on the tropical year (the year of the seasons) whose length at that time had been determined to be 365.25 days. To compensate for the extra quarter of a day, he set up a leap year system. Common years had 365 days and every fourth year was a leap year with 366 days.
By the sixteenth century, astronomers realized that Caesar's year length, 365.25 days, was 11 minutes and 14 seconds too long. This may seem trivial, but after a few centuries the difference added up to 10 days. As a result, the vernal equinox was no longer on March 21st and this meant that Easter was being celebrated at the wrong time.
In 1582, Pope Gregory XIII instituted a calendar reform. Ten days were dropped from the calendar so that the vernal equinox would be brought back to March 21st. The day that followed October 4th that year was October 15th. They also changed the leap year rule. Under the Julian system, every fourth year was a leap year. That was too many. The Gregorian leap year rule is that every fourth year is a leap year unless it is a century year. Century years must be evenly divisible by 400 to be leap years. Thus 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not.
The Roman Catholic and Protestant churches now determine the date for Easter according to the Gregorian Calendar, but Orthodox Christians still follow the Julian Calendar. That is why they celebrate Easter at a later date.