You should. Christmas was celebrated on December 25th hundreds of years before there was a King of England.
The 1st Recorded Celebration of Christmas
http://www.christianity.com/church/church-history/timeline/301-600/the-1st-recorded-celebration-of-christmas-11629658.html
According to Clement of Alexandria, who gave a then-synopsis of what Christians then believed was the actual days of the Lord’s birth 9and they were much closer to His birth than we are), Christians then placed Jesus’ birth on March 21, April 21, April 20 or 21.
By looking at and citing Clement (and the first-hand account that he had, and thus avoiding the historical revisionism which we see today), the consensus before and at Clement’s time was March or April, with the date of 21 (March or April) being the most recognized date.