The CENTCOM Commander decides H-hour. It could be day/night/moon/no moon. There's a wide variety of factors to consider and numerous courses of action to take. His staff briefs him several times daily on all aspects of the operation. Whatever decision he makes will be tempered by the the best course of action that meets his intent, ensures success and mitigates risk against the enemy's most probable course of action against our forces. The staff process to get to a decision is analytical and painstaking making his decision problematic in nature.
The short answer: They will go when they go. We'll read about the whys after the fact.