Easter letter

The Festal Letters or Easter Letters are a series of annual letters by which the Bishops of Alexandria, in conformity with a decision of the First Council of Nicaea, announced the date on which Easter was to be celebrated. The council chose Alexandria because of its famous school of astronomy, and the date of Easter depends on the spring equinox and the phases of the moon.

The most famous of those letters are those authored by Athanasius, a collection of which was rediscovered in a Syriac translation in 1842. Festal Letters of other Bishops of Alexandria, including Cyril have also been preserved.

Of the 45 Festal Letters of Athanasius, the 39th, written for Easter of AD 367, is of particular interest as regards the biblical canon.

In this letter Athanasius lists the books of the Old Testament as 22 in accordance with Jewish tradition. To the books in the Tanakh he adds the Book of Baruch and the Letter of Jeremiah, but he excludes the Book of Esther.

He lists the books of the New Testament as the familiar 27: the 4 Gospels, the Acts of the Apostles, the 7 General Epistles (listed in the order in which they appear in modern editions of the New Testament), the 14 Pauline Epistles (listed with the Letter to the Hebrews placed between those to the Thessalonians and the Pastoral Epistles), and the Book of Revelation. Although the order in which Athanasius places the books is different from what is now usual, his list is the earliest reference to the present canon of the New Testament.

...
Wikipedia