What caused Washington to attack treton on Christmas night?

I think because early protestants like the colonist didn't celebrate Christmas, unlike the Catholic German mercenaries on the other side of the Delaware, who were drinking and celebrating heartily.

To the best of my knowledge, Protestants did not and some may still not, observe December 25th as the actual historic birth-date of Christ, so it was just another day for them, and an Excellent time to attack a hung over and unwary enemy.