ATHENS, Greece (AP) — For more than 400 years, Catholic and Orthodox churches have used different ways to determine the date of Easter. But this Easter marked a special moment for Christians, as the churches celebrate of Jesus’ resurrection on the same day.

What’s more, top religious leaders — including Pope Francis — are expressing a desire to keep it that way. But the unusual alignment has stirred underlying mistrust between the two major Christian communions.

The movable date for Easter follows a seemingly straightforward rule: the Sunday following the first full moon on or after the spring equinox. But the two churches started using different calendars after Pope Gregory XIII’s adaptation in 1582, when the Western church adopted the Gregorian calendar while the Eastern Orthodox Church kept the older Julian one.

Moreover, each church uses its own ecclesiastical calculations for lunar cycles and the equinox, which don’t neatly match scientific projections.

The result is that Easter dates can be as much as five weeks apart. They can coincide in back-to-back years, or a decade can pass without it happening.

POPE FRANCIS’ WISH

Days before his five-week hospitalization, Pope Francis referred to this year’s Easter celebration while invoking the 1,700th anniversary of the historic Council of Nicaea, when Christian leaders gathered to settle foundational disputes about the faith.

“Once again, I renew my appeal: Let this coincidence serve as a sign — a call to all Christians to take a decisive step toward unity around a common date for Easter,” Francis said while leading prayers at the Basilica of St. Paul in Rome.

Francis’ invitation, delivered at the end of a prayer for Christian Unity with Orthodox priests present, wasn’t new. Returning from a trip to Turkey in 2014, he told reporters on the plane that a unified date would be logical.

“It is a bit ridiculous,” he said, then staged a pretend conversation: “‘Tell me, your Christ, when is he resurrected? Next week? Mine was resurrected last week.'”

He has found an ally in Ecumenical Patriarch Bartholomew, a fellow octogenarian and spiritual leader of the world’s Orthodox Christians. The two “speak to one another like brothers,” Francis has said. For his part, Bartholomew has called Francis “our elder brother” and described the Easter initiative as “a real step toward repairing old conflicts.”