What year was December 25th established as Christmas?

The earliest evidence of Christ's birth being marked on December 25 is a sentence in the "Chronograph of 354." Liturgical historians generally agree that this part of the text was written in Rome in AD 336. Though Christmas did not appear on the lists of festivals given by the early Christian writers Irenaeus and Tertullian, the early Church Fathers John Chrysostom, Augustine of Hippo, and Jerome attested to December 25 as the date of Christmas toward the end of the fourth century.