You negotiate the best possible outcome you can negotiate that makes both parties ok with the situation. If you cannot do that, then you have to take it back to court. For example, mom notified you in advance that she needed that weekend. So, what I would have done is negotiated back and forth about which weekend you would take within a two month spread to make up for the missed weekend.
If it was a holiday then I would decide whether or not to say no and stick to the guidelines based on why she wanted the holiday. For example, if she wanted to visit her elderly grandmother for Christmas because grandma wasn't doing very well, but could not do so because of the somewhat screwy way that the guidelines divide Christmas, then I probably would have agreed to trade Christmas day for another holiday, and celebrated my Christmas with the kids on one of the other days during Christmas break. In fact, I know of many families in Indiana who throw out the Christmas Holiday Schedule and just divide Christmas break in half, flip flopping the first and second week every other year.
However, if she wanted Christmas Day for some trivial reason, then I might say no.