25th December 354 CE.
The Anglo-Saxons had their winter festival and the Norse had Yule, both of which traditions have survived into our secular age.
MERRY CHRISTMAS TO ALL MY READERS.
In the 7th century the yearly calendar was reckoned from Christmas Day, until the 12th century, when the anniversary of the Annunciation, March 25th, became widespread throughout the Catholic world.
The only remnant of this calculation is that the tax year in Britain now begins on April 6th, resulting from the loss of 11 days from the new Gregorian Calendar of 1752.
In an old list of Roman bishops dated Today in 354 we read: ‘AD 336 25 Dec natus christus in Betleem Judeae’, which constitutes the first known reference to the celebration of Christmas Day.(1)
Within half a century this had become an important date in the Catholic Christian year with the 25th fixed as the ‘Natural’ (birth) Date, with the aim of exorcising the early pagan festival of solstice, which in fact continues in a growing Pagan tradition to this day.
When we use the short term ‘Xmas’ we are looking back to an ecclesiastical abbreviation first used in tables and charts which referred to the first letter of Christ in the Greek letter Chi, identical to our X.
It later became the custom not to stress Births as Baptisms were more regarded, until relatively modern time, so in medieval Britain, January 6th Epiphany (baptism of Jesus), was the main celebration, rather than Christmas.
Ref: (1) First mentioned in the Roman Calendar Chronographus Anni CCCLIV Chronographer (the year 354).