It's a points based system.
Each day is signed a set number of points, which is then multiplied depending on whether you bid for it, it's assigned or you take it in the open phase.
Then every year around November the bid opens for the following year, where you are in the priority depends on the points you have.
For normal captains, summer, Christmas and half terms etc are high points (busy periods for the company).
So, the theory is you will get summer/christmas etc every other year.
But it depends heavily on how you bid. It's quite a good fair system that a lot of people don't like.
However, if you want school holiday leave the best bet is to take parental leave!