How exactly December 25th came to mean the Christmas we all know today is unclear. No specific dates exist in any of the literature, and it is thought the date may correspond to the nine month period after the Annunciation, when Jesus was conceived, as well as to the Roman pagan midwinter festival.
In any case, documented commemorations of the birth of Jesus existed from the fourth century A.D. Americans celebrated the holiday since the earliest settlers, but for various reasons did not make it a federal holiday until a century after the independence. On June 26, 1870, after religious hard lines and anti-British sentiment quelled, Congress declared Christmas a federal holiday.
The early Pilgrim settlers of America considered any form of celebration sacrilegious; more so even than their Puritan brethren in England, who considered the holiday little more than a festival of decadence. That feeling was intensified by the anti-British fervor that swept over the colonies during the Revolutionary War and the early years of the colonies.