When was the earliest Christ Mass performed ever?
According to historian Bruce David Forbes in Christmas: A Candid History:
"Early Christians had no Christmas. The first written evidence of an annual celebration on December 25 commemorating the birth of Jesus comes from the fourth century. Epiphany developed a little earlier, in the eastern portion of the Christian church, but it was not quite the same as what we now call Christmas. Compared to many other aspects of early Christianity, Christmas was a later development" [pg. 17].
"The earliest celebrations of Epiphany arose in eastern Christianity, although even that claim is debated by a few scholars. The fact is, our knowledge about the earliest forms of Epiphany is very sketchy. The noted historian Roland Bainton began a classic article on the topic by stating simple, "The origins of the Christian feast of Epiphany on the sixth of January are still obscure," and unfortunately, very little has been clarified in the years since he wrote those words" [pg. 22].
"The earliest indication of what might be called Epiphany festivities comes in comments by Clement of Alexandria (approximately 150-215), who mentioned that the 'followers of Basilides' in Egypt celebrated Jesus' baptism on January 6. This group, although they saw themselves as Christians, were eventually considered heretics by the majority church" [pg. 23].
"In the West, the first written record we have that associates the birth of Jesus with December 25 is found in a Roman document called the Philocalian Calendar, also known as the Chronograph of 354, which is itself a collection of lists and records, something like an illustrated almanac. Its information was drawn from both civil and Christian sources, and it included, among other things, lists of Roman holidays, a table of dates for Easter, lists of Roman bishops and Christian martyrs, with their burial locations, a listing of the consuls of Rome, ... At two points in this document, December 25 was cited as the birth date of Christ" [pgs. 24-25].
"Later in the fourth century, sermons and other church documents provide additional evidence that Christians were celebrating December 25 as the birth of Christ and that the practice was spreading throughout the larger church" [pg. 25].
According to the Catholic Encyclopedia, "In Cappadocia, Gregory of Nyssa's sermons on St. Basil (who died before 1 January, 379) and the two following, preached on St. Stephen's feast (P.G., XLVI, 788; cf, 701, 721), prove that in 380 the 25th December was already celebrated there ..." [Christmas].