Transferred from the 6th of January to the 25th of December by Julius I. (337-352). Old Christmas Day. January 6th. When Gregory XIII. reformed the Calendar in 1582, he omitted ten days; but when the New Style was adopted in England in 1752, it was necessary to cut off eleven days, which drove back January 6th to December 25th of the previous year. So what we now call January 6th in the Old Style would be Christmas Day, or December 25th.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894