VATICAN CITY, APR. 13, 2001 ( Eastern and Western Christians and Catholics are celebrating Easter on the same day this year, and John Paul II wants to highlight that fact in the Mass he celebrates this Sunday.

His Easter Mass will have elements of the Byzantine liturgical tradition. The medieval custom of the Roman Church will be relived, singing the "stichi" of the Byzantine Easter liturgy before the Pope, according to Bishop Piero Marini, the master of pontifical liturgical celebrations.

The last time Eastern and Western Christians celebrated Easter together was in 1990. The difference in the date of the celebration arose after Pope Gregory XIII´s reformation of the liturgical calendar in 1582. Eastern Christians, in the majority Orthodox, continue to calculate the date of Easter according to the old Julian calendar, established by Julius Caesar in 46 B.C.

The "stichi" are verses of Psalm 67 (68), interspersed with poetic verses that sing of Jesus´ resurrection and of Christian joy. Their present version dates back to the sixth and seventh centuries. They reflect theological themes of the homilies of the Eastern Church Fathers, such as John Chrysostom, Basil the Great, Gregory Nazianzen, and even older texts, including second-century Easter homilies.

"Among all Christians there is a growing desire to reach a common celebration of Easter on the same date," Bishop Marini said, in a press statement. "In the appendix to the liturgical constitution ´Sacrosanctum Concilium,´ Vatican Council II expressed this desire. In an ecumenical dialogue held in Aleppo, Syria, in the month of March 1997, some hypothesis were presented in this respect."

The Psalm and poetic verses will be proclaimed before the Holy Father after the reading of the Gospel. The choir will sing a Russian melody composed by Grigori Lvovski (1830-1894), which harmonizes these passages.

"In this way, the Eastern and Western singing, the paschal sequence of the Roman liturgy, and the Eastern hymn of the Byzantine liturgy will proclaim together faith in the Lord´s resurrection," Bishop Marini explained.

This is a symbolic gesture which responds to the Pope´s great concern expressed in the apostolic letter with which he concluded the Jubilee, "Novo Millennio Ineunte."

"In the perspective of our renewed post-Jubilee pilgrimage," he says in section No. 48 of the letter, "I look with great hope to the Eastern Churches, and I pray for a full return to that exchange of gifts which enriched the Church of the first millennium. May the memory of the time when the Church breathed with ´both lungs´ spur Christians of East and West to walk together in unity of faith and with respect for legitimate diversity, accepting and sustaining each other as members of the one Body of Christ."