Why does washington need to attack before december 31st?

Washington needed to attack before December 31st because the Hessian soldiers would have become alert to face an attack from them after their Christmas and New Year celebrations were over.

It was the part of the war strategy formulated by Washington to attack the Hessian army before 31st December. This was because he wanted the Hessians to be engaged in the celebrations of Christmas and New Year which would make it easy for the American troops to defeat them.

did you check the study guide?