Use of CE and Ces

«ce sont» is the plural form of «c'est». «ce» used in this manner is an indefinite, invariable pronoun that is used mostly, but not exclusively, with «être» (instead of cela/ça). 

Of course «ce» can also be an adjective - «ces» is then the plural form,  and is followed by a noun, not a verb.

C'est, ce sont = this is, these are (demonstrative pronouns)

demonstrative-adjectives-ce-cet-cette-and-ces-mean-this-that-and-these-those

That are relaxing holidays. -- Ce sont des vacances relaxantes.
These holidays are relaxing. -- Ces vacances sont relaxantes.

Notice how, in the first sentence, ce does not refer to vacances but is a standalone subject of the sentence. In the second example, ces is a demonstrative pronoun belonging to vacances and needs to be matched to it.

That explanation was so enlightening. It is wonderful  when things make sense.