The Formula of the Council of Nicaea
We know that Easter must always occur on a Sunday, because Sunday was the day of Christ's Resurrection. But why the paschal full moon? Because that was the date of Passover in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, Easter was the Sunday after Passover.
The Church does not use the exact date of the paschal full moon but an approximation, because the paschal full moon can fall on different days in different time zones, which would mean that the date of Easter would be different depending on which time zone you live in. For calculation purposes, the full moon is always set at the 14th day of the lunar month (the lunar month begins with the new moon). Likewise, the Church sets the date of the vernal equinox at March 21, even though it can occur on March 20. Both approximations allow the Church to set a universal date for Easter.
Why Do Eastern Orthodox Celebrate Easter on a Different Date?
Still, Easter isn't celebrated universally on that date. While Western Christians use the Gregorian calendar (the calendar that's used throughout the West today, in both the secular and religious worlds) to calculate the date of Easter, the Eastern Orthodox continue to use the older, astronomically inaccurate Julian calendar. Currently, March 21 on the Julian calendar falls on April 3 in the Gregorian calendar. Therefore, for the Orthodox, the Sunday following the 14th day of the paschal full moon has to fall after April 3, hence the discrepancy in the date of Easter.
How Is the Date of Easter Related to Passover?
Many Christians believe that the date of Easter is determined by the date of Passover, and so they are surprised when, in years such as 2008, Western Christians celebrate Easter before the Jewish celebration of Passover. (See Reader Question: Why Does Easter Come Before Passover This Year?)
But look closely once again at the formula for calculating Easter, established at the Council of Nicaea: Easter is celebrated on the Sunday following the paschal full moon. Notice what isn't mentioned? That's right: Passover.
In fact, the reason the Council of Nicaea set up a formula for calculating the date of Easter was to separate the Christian celebration of Christ's Resurrection from the Jewish celebration of the Passover. Some Christians in the early fourth century were calculating the date of Easter according to the Jewish calendar, while others believed it needed to be calculated separately.
The Council of Nicaea agreed with the latter group. While Easter and Passover were related historically—as I noted above, the Last Supper occurred on the Passover—and theologically, the Council declared that the connection between the two holidays ended with the Death and Resurrection of Christ.
Why? Because Christ is the new Paschal Lamb, of Whom the lamb sacrificed at Passover was just a sign or symbol. That symbol found its completion in the sacrifice of Christ on the Cross. Thus, the continued celebration of the Jewish Passover has no theological significance for Christians, because Christ is our Passover Lamb.
But Don't the Eastern Orthodox Calculate Easter With Reference to Passover?
Still, many Christians remain confused on this point, because currently, the Eastern Orthodox celebration of Easter always occurs after the Jewish celebration of Passover. Therefore, they have assumed that the Eastern Orthodox calculate the date of Easter differently, and that the Orthodox Church include in its calculation a consideration of the modern date of Passover.
In fact, however, the Orthodox use the very same formula established by the Council of Nicaea. As the Antiochian Orthodox Christian Archdiocese of North America explained in a 1994 article entitled "The Date of Pascha," the modern date of Passover has nothing to do with the Orthodox date of Easter. The difference, as the article explains, is entirely the result of the Orthodox Church's continued use of the astronomically inaccurate Julian calendar. (For more details, see Reader Question: Is the Date of Easter Related to Passover?)