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.
The 39th Festal Letter of Athanasius
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.
Athanasius reckons, not as part of the canon of Scripture, but as books "appointed by the Fathers to be read by those who newly join us, and who wish for instruction in the word of godliness": the Book of Wisdom, Sirach, the Book of Esther, Judith, the Book of Tobit, the Teaching of the Apostles, and the Shepherd of Hermas. Despite this distinction, J. Leemans has argued that there is no difference in the way Athanasius uses these books and the way he uses those he designated as in the New Testament. For example, in his writing "Against the Arians", Athanasius cites Sirach 30:4 as "Holy Scripture". 
In addition to the books that he calls either canonical or books to be read, he speaks also of books to be rejected, calling them apocrypha, and describes them as "an invention of heretics, who write them when they choose, bestowing upon them their approbation, and assigning to them a date, that so, using them as ancient writings, they may find occasion to lead astray the simple".
- "NPNF2-04. Athanasius: Select Works and Letters".
- William Cureton (editor), The Festal Letters of Athanasius (Society for the Publication of Oriental Texts, London, 1848)
- Ebied, R. Y.; Wickham, L. R., eds. (1975). A Collection of Unpublished Syriac Letters of Cyril of Alexandria (Corpus Scriptorum Christianorum Orientalium). Louvain. p. 52. ISBN 978-9-04290401-9. Retrieved 8 January 2021.
- An English translation of the relevant part of the letter is available in the Christian Classics Ethereal Library.
- Livingstone, E. A.; Sparkes, M. W. D.; Peacocke, R. W., eds. (2013). The Concise Oxford Dictionary of the Christian Church. Oxford University Press. p. 90. ISBN 978-0-19965962-3.
- Auwers, Jean-Marie; de Jonge, H. J., eds. (2003). The Biblical Canons. Peeters Publishers. p. 267. ISBN 978-90-4291154-3.
- "Apologia Contra Arianos, Part II (Athanasius)". New Advent.
- "From Letter 39 (Athanasius)". New Advent.