Why was Christmas chosen as the attack day? (The war of George Washington)

The Battle of Trenton happened December 26, 1776 washington and troops crossed the deleware river with 18 cannons. after crossing they marched 19 miles south for a suprise attack on the british troops in trenton, the colonists won the battle.

The attack was unsuccessful, but it was just a way to catch everybody off guard (like a surprise attack).