I have celebrated a Christian Passover seder for a couple years now. We usually do it the Thursday before Easter, which is not the same date as the Jewish Passover. I began wondering why the date for Easter does not correspond with the Jewish Passover, so like a good librarian, I've done a little digging!
The Hebrew calendar is a "lunisolar" calendar, meaning that the months are determined by the phase of the moon, and every few years an extra month is added to keep the seasons in the same place of the year. This way, the Hebrew year always begins in the Spring.
The calendar in use by the subjects of the Roman Empire at the time of the early Christians (and up until the 16th century) was the Julian calendar, a "solar" calendar. The calendar we use today, the Gregorian calendar is also a solar calendar. However Jewish holidays were (and still are) determined by the Hebrew calendar. The Jewish Passover is celebrated on the 15th day of Nisan but this day would move around in relation to the Julian calendar. Sometimes it could be on Tuesday, sometimes Thursday, etc. The early Christians had established Sunday (of the Julian calendar) as the day of the week on which to gather and worship and wanted to keep this tradition for celebrating the Resurrection. Those who were Jewish also kept the Passover, as determined by the Hebrew calendar.
Many early Christians, beginning with the apostles had a tradition of celebrating the Resurrection on the first Sunday following Passover. Over time, the Passover lost importance in the Church. Gentiles had never celebrated it to begin with and others felt that the celebration of Easter replaced that of the Passover.
In 324 the Council of Nicaea convened. This is seen as the beginning of the "Catholic" Church. One of the items they covered was establishing an official date for Easter. When determining this they did not consider the date of the Jewish Passover. They created a complex formula for determining the date of Easter each year, but in the simplest terms Easter was to be celebrated on the first Sunday after the first full moon of the vernal equinox. The vernal equinox that year was March 20th.
Over time, the Julian calendar got out of sync with the seasons. In 1582 the Julian calendar was replaced with the Gregorian calendar, making some adjustments and adding "Leap Year". However, the Catholic Church continued to determine the date of Easter the same way it had been. As the Protestant Church came out of the Catholic, we continue to celebrate Easter on the same day.
And so, this is why the date of Easter has nothing to do with the date of Passover!!!
