Patriarch Kirill of Moscow and All Russia sent Christmas greetings to the heads of Christian denominations celebrating the Nativity of Christ on December 25 according to the Gregorian calendar.

In an official address published on the website of the Department for External Church Relations (DECR) of the Russian Orthodox Church, Patriarch Kirill conveyed warm wishes, extending heartfelt congratulations on the great feast of the Nativity of Our Lord Jesus Christ. He wished strength and abundant divine assistance in their continued high ministry.

The greetings were addressed to Pope Francis, Patriarch Mor Ignatius Aphrem II of Antioch and All the East, head of the Syriac Orthodox Church; Catholicos-Patriarch Mar Awa III of the Assyrian Church of the East; Cardinal Bechara Boutros al-Rahi, Maronite Patriarch of Antioch and All the East; and Catholicos Baselios Marthoma Mathews III, Metropolitan of the Malankara Orthodox Syrian Church.

The message highlights that the difference in the dates of Christmas celebrations—December 25 and January 7—is due to the use of different liturgical calendars: the Gregorian, Julian, and Revised Julian calendars.

The Julian calendar was introduced in 45 BC by Emperor Julius Caesar and included leap years, resulting in an average year length of 365.25 days. The Gregorian calendar was established in 1582 by Pope Gregory XIII to correct the accumulated error in determining the date of the spring equinox.

In 1923, during the Pan-Orthodox Congress in Constantinople, presided over by Patriarch Meletius IV, the Revised Julian calendar was adopted, which aligns with the Gregorian calendar until the year 2800.

Currently, the Gregorian calendar is used by the Catholic Church and most Protestant churches. The Julian calendar is maintained by the Russian, Serbian, Jerusalem, and Georgian Orthodox Churches, while the Revised Julian calendar is observed by the Constantinople, Cyprus, Greek, Polish, Romanian, Alexandrian, Antiochian, Albanian, and Bulgarian Orthodox Churches.