I wonder if there's a definitive answer to this ... two of these say 336, one says 333, and another says 354:-
Christmas (i.e., the Mass of Christ) was not among the earliest festivals of the church. According to a Roman almanac, the Christian festival of Christmas was first celebrated in Rome on December 25th in AD 336. Prior to this, throughout the Roman World, the Saturnalia from 17th December was a time of merrymaking and the exchange of gifts.
http://messianicjudaism.homestead.com/christmas.html
he birthday of YAHSHUA (Jesus) was never observed by the apostles or the early Messianic Faithers. His birthday was first celebrated by the Roman Catholic Church in 333 AD.
domaintrader2000.com/dov-xmas.htm [no longer exists]
By the year 273, most leaders settled on December 25 as the natal day, most likely because two other festivals were already being celebrated that day ... Christmas was first celebrated on December 25 in 336, after Emperor Constantine had declared Christianity the Roman empire's favored religion. For a while, Eastern churches, however, celebrated January 6 as the date for the birth of Jesus and his baptism. Most Eastern Orthodox churches eventually adopted December 25, celebrating the birth on the earlier date and his baptism on the latter, but the Armenian church still celebrates his birth on January 6.
newcelebrations.com/newsletter_5.htm [no longer exists]
https://www.biblicalarchaeology.org/daily/people-cultures-in-the-bible/jesus-historical-jesus/how-december-25-became-christmas/