This blog has touched on the debate over the date of Easter in the past, but the truth is that the early Church went through different phases before settling on the date of Easter.
Because the Last Supper was a seder, commemorating Passover, early celebrations of Easter coincided with that date. Passover took place on the 14th of the month. The early Church historian Eusebius tells us that the dioceses of Asia at the time of Pope Victor (pope from 189-199) celebrated Easter on the 14th day of the moon, regardless of the day of the week on which it fell.
This bothered some ecclesiastics and Christian scholars. Synods were held (Eusebius says) that agreed and decreed that the Easter celebration should be held on the Lord's Day, a Sunday. Some, however, refused to give up the tradition of celebrating on the 14th. They were called Quartodecimans [fourteenth-ers]. St. Polycarp (69-155), for example, came to Rome to discuss his preference for the date that he believed had been established by St. John the Apostle; he refused the command of Pope Anicetus (pope c.153-168) to change to Sunday.
Quartodecimans were tolerated for awhile, by popes like Anicetus at least. Pope Victor excommunicated the Asiatic dioceses, an action that got him criticism for unnecessary harshness from St. Irenæus.
Agreeing that Easter should be celebrated on a Sunday did not settle any debate; which Sunday was crucial. The Council of Nicaea (already mentioned several times in DM) tackled this issue. Syrian Christians always celebrated Easter on the Sunday following the 14th of the month, but other Christian dioceses calculated the date in their own ways. Antioch, for instance, based their date on the local Jewish observances, but had let slide a guideline that the 14th should be the month after the vernal equinox. Alexandria, however, demanded Easter Sunday be after the equinox—March 21st at the time.
Most native English speakers, if they know about the controversy of the Easter date debate, have heard of the Synod of Whitby in 664, at which Roman Christianity and Celtic/Irish Christianity fought it out over topics such as the date of Easter and the style of monastic tonsures. Whitby established for the English-speaking world that Easter would fall on the first Sunday after the first full moon (the 14th of the lunar month) after the vernal equinox. If the full moon is a Sunday, Easter takes place on the following Sunday. Easter can be as early as March 22 or as late as April 25.
The Eastern Orthodox Church calculates differently. They had been using March 21st as their starting point, but followed a guideline that prevented Easter from ever falling on or preceding the same day as Passover. Orthodox Easter can fall between April 5 and May 8. In the 21st century, the Roman Catholic and Orthodox churches tried to reconcile their different dates using more recent astronomical data for their calculations. They still calculate in different ways, but there is greater chance that the dates will coincide, such as in 2001 when April 15th was Easter for both Churches.
There. That was easy.