GENEVA , April 9 (AP) -- Christians everywhere will celebrate Easter on the same day this year because of a calendar quirk, a coincidence that has revived interest in trying to set a universal date for the observance.

Both Western and Eastern churches agree that the date should be based on a principle set in the year 325, which states that Easter falls on the Sunday following the first full moon after the spring equinox.

However, the dates vary because Protestant and Catholic Churches follow the 16th-century Gregorian Calendar, while the Orthodox churches use the older Julian Calendar. The two currently differ by 13 days.

Easter can occur between March 22 and April 25 for the Western Christian churches, while the range for Orthodox Easter extends from April 4 to May 8. ``Especially in regions where Christians of the Western and Eastern traditions live closely together and may even constitute a minority, as for example in the Middle East, this situation is extremely painful,'' said a statement Monday from the World Council of Churches. The council includes non-Catholic Christian churches, including Orthodox faiths, from across the world.

This year, by chance, both calendars set the same day for the spring equinox and the full moon following it, meaning that Eastern and Western churches will celebrate Easter on the same day.

Church leaders have pondered the idea of agreeing on a single date for Easter for decades. In 1975, the Roman Catholic Church proposed that the date should be fixed on ``the Sunday following the second Saturday of April.'' The idea was later dropped because it could not be accepted by all Christian faiths.

In 1997, at a meeting in Aleppo, Syria, participants agreed that Easter should be set according to the method established in 325, using accurate astronomical data to establish the date of the spring equinox.

Under this system, the date of Orthodox Easter would change in most years. For churches using the Gregorian Calendar, the first year in which the date would be affected is 2019.

``But for some Orthodox, the calendar is so closely related to the tradition that changes are unthinkable,'' said Dagmar Heller, of the Evangelical Church in Germany.

There's no agreement on the horizon, but the issue will likely be revisited over the next 20 years - during which the Easter dates will coincide six times. The problem has already been solved in Finland, where the tiny Orthodox Church was given permission in the 1920s to celebrate Easter according to the Gregorian Calendar.

``This has been a great blessing for our small minority church in a Protestant country,'' said Metropolitan Ambrosius of Oulu, of the Finnish Orthodox Church.

``We have been able to bear common witness to the mystery of the Resurrection. It makes us stronger.''