General rule of thumb, the prices all go up while school is out and go back down when school starts again. This is true for the Summer Vacation, plus the Spring and Christmas breaks. Since not all schools have the same schedule the prices are not quite as bad the first and last couple of weeks.
This applies to Caribbean cruises since we haven't cruised North or in Europe so I don't know how those prices fluctuate.
We've found the best prices for the Caribbean cruises to be September and October. Not too bad in late January and February.