Ce sont des vacances. -- These are holidays. Pendant ces vacances je m'ennuyais beaucoup. -- On these holidays I used to be bored a lot.
In the former construction "ce" does not refer directly to holidays. It is the subject of the sentence all by itself.
In the second sentence "ces" goes with holidays like any adjective does with its verb. And since "vacances" is plural, "ces" is plural as well.
