So, I'm quite a novice, and I'm having some difficulty parsing the sentence:
I "understand" it means "Summer vacation's already over". I also understand 夏休み means summer vacation.
At this point, I am speculating, so please tell me if I make any errors: もう終わりだ is equivalent to もう終わりです, which means, it's (already?) over. However, I am confused at the use of も as the particle preceding もう終わりです. The only use I know for も is when replacing を or は to signify "also".
Could someone please clarify the use of the particle も and the meaning of もう in もう終わりだ.