My husbands ex has known about our vacation for months, she is now requesting her weekend possession the day we leave for vacation. What can be done.

Generally speaking, unless the parents reached an agreement otherwise, they should abide by the possession schedule set forth in their court order.One parent can't preempt another parent's possession time by scheduling a vacation that conflicts, even if they do so with advance notice.With respect to summer vacations, the standard possession order provides that each party can give written notice in advance of selected periods for the exercise of their respective summer periods of possession which supersede any conflicting weekend periods of possession set forth in the order.  So each parent typically has an opportunity under the standard possession order to plan and take a summer vacation with the child/children.  When a parent wants to take an extended vacation with the child/children outside the parameters of that summer time frame, it is incumbent on the parent to approach and reach an agreement with the other parent.