Motorists across America saw a strange sight this past Sunday morning if they stopped at a traffic signal near an Eastern Orthodox sanctuary and then, shortly thereafter, passed a Catholic parish. What they saw was worshippers singing hymns and waving palm fronds as they marched in Palm Sunday processions at these churches. Similar sights will be common during Holy Week rites this week and then on Easter Sunday.

There is nothing unusual about various churches celebrating these holy days in their own ways. What is rare is for the churches of the East and West to be celebrating Easter ("Pascha" in the East) on the same day. This will happen again next year, as well as in 2014 and 2017.

This remains one of the most painful symbols of division in global Christianity. While Easter is the most important day on the Christian calendar, millions of Christians celebrate this feast on different days because they have -- for centuries -- used different calendars. The Orthodox follow the ancient Julian calendar when observing Pascha, while others use the Gregorian calendar introduced in 1582, during the reign of Pope Gregory XIII.

"It was a calendar issue then and it's a calendar issue now," said Antonios Kireopoulos, an Orthodox theologian who is a leader in interfaith relations work at the National Council of Churches of Christ. "This is about calendars, but it's much more than that."

This clash between liturgical calendars in the East and West, he said, also affects how churches pursue their missions. "We are talking about the central event of our faith, yet we remain so divided about it. ... That has to raise questions for those outside the faith. If the resurrection is so important, why can't we find a way to celebrate this together?"

Seizing the temporary unity represented by the shared Easter dates this year and next, Kireopoulos and National Council of Churches General Secretary Michael Kinnamon recently renewed an earlier call that challenged leaders on both sides to pursue a permanent solution to this clash of the calendars.

Their letter restates three recommendations from the 1997 Aleppo Conference, which was hosted by the Syrian Orthodox Church of Antioch. That gathering called for Christians worldwide to:

* Honor the first ecumenical council of Nicea by celebrating Easter on the first Sunday following the first full moon after the spring equinox, which would maintain the biblical ties between the Jewish Passover, Holy Week and Easter.

* Agree to calculate astronomical data by using the best available scientific methods, which was a principle established in Nicea to settle an early controversy about the date of Easter.

* Use the meridian line for Jerusalem as the reference point for all calculations, once again honoring the biblical narratives about the death and resurrection of Jesus.

The problem, of course, is that making a change of this magnitude would require a broad spectrum of Christian leaders -- including the pope and numerous Orthodox patriarchs -- to agree on something that stirs deep emotions among the faithful. Orthodox leaders continue to wrestle with splits linked to a 1923 decision to celebrate Christmas according to the Gregorian calendar.

The final Aleppo document recognized that it would be especially hard for Eastern believers to change their traditions.

"In some countries in the Middle East and Eastern Europe, where the Christian churches have lived with the challenge of other religions or materialistic ideologies, loyalty to the 'old calendar' has been a symbol of the churches' desire to maintain their integrity and their freedom from the hostile forces of this world," it said. "Clearly in such situations implementation of any change in the calculation of Easter/Pascha will have to proceed carefully and with great pastoral sensitivity."

Orthodox leaders know that the Easter gap will keep getting wider -- with Pascha creeping into the summer in about a century.

But change is hard. As old joke says, "How many Orthodox Christians does it take to change a light bulb?" The answer: "Change? What is this 'change'?"

"This is not a matter of one side finally giving in and the other winning," stressed Kireopoulos. "This is a matter of finding a way to proclaim -- together -- what we all believe about the resurrection of Jesus Christ. ... What we hope is that, once again, we can follow the principles of Nicea and find a way to move forward."