Why does Washington need to attack before December 31, 1776

Many of washingtons men would no longer be obligated to fight due to the provisional enlistments that typically lasted for no longer than a year. Many of them had already decided they had done their part and since moral was low at this time, it was expected a lot of men would stop fighting on January 1st. Because of this, he wanted to attack while he still had the men to have more power and also boost morale among the men who would potentially be leaving