In the contemporary Western world, Christmas starts with Christmas Eve on Dec. 24. and ends with Boxing Day on Dec. 26. In times now long past, though -- and on calendars now long since consigned to history -- the date of Christmas and celebrations of the birth of Christ have varied from Dec. 25 to Jan. 6 and April 19 -- and even to as late in the year as May 20.
The Western Church, which arose in the Roman Empire, probably settled on Dec. 25 because it was already invested with meaning as the Saturnalia, the Roman celebration of the great winter solstice, as well as being a day of celebration for devotees of Mithras, the Roman sun god.
The winter solstice was also a day of celebration for the Celts and other northern folk. It was from the Celts' ritual use of mistletoe that the parasitic plant was incorporated as a Christmas decoration back in merrie olde Tudor England (1485-1603), when Christmas was celebrated for 12 days, with the festivities concluding on Jan. 6, known as Twelfth Night.