Why Catholic and Orthodox Christians Celebrate Easter on Different dates?

Easter is one of the most important holidays for Christianity. It is called a moveable feast as it falls on different dates and every year it is different from the previous one. And why is it like this? Why is it so hard to track when it should be celebrated?

The Orthodox Church celebrates Easter on the first Sunday following the first full moon of the year after the Spring Equinox. And according to the Ecclesiastical tables, the first full moon is the first Ecclesiastical full moon date after March 20 and this date can vary from the date of actual, astronomical, full moon. Thus, Easter can range from March 22 through April 25 in the Orthodox calendar. In addition to this, the Orthodox use the old Julian calendar to calculate the date of Easter and Catholics use the new Gregorian calendar. In both calendars Easter and holidays related to it do not have a fixed date. According to the current calendar methods – using the Julian and the Gregorian calendars – Orthodox and Catholic churches celebrate Easter on the same date once every 3-4 years. The Gregorian calendar differs only slightly from the Scientific Astronomical calendar while the Julian calendar is currently about 13 days off of the Scientific Astronomic calendar. That is why Catholics will face only one change in the dates of Easter over the next 25 years, and Orthodox will face 17 changes in the date of Easter over the next 25 years.  

Since the Orthodox use the Julian calendar, their Easter can only be celebrated after the Jewish Passover while Catholics have another story: according to their calendar Easter should be celebrated before the Jewish Passover. That is the most probable reason for celebrating Easter on different days.

While it is not important when we celebrate it, the reunion of people of different religions plays a most important role in our life.