Particle「も」in the sentence 「夏休みももう終わりだ」。

So, I'm quite a novice, and I'm having some difficulty parsing the sentence:

「 夏休{なつやす}みももう終{お}わりだ。」

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.

Answer

夏休みももう終わりだ。

According to 明鏡国語辞典:

も 🈩副助
⓭ 軽い詠嘆を込めて物事をとりたてる。多く、人為を超えた物事の順当な推移についていう。
「夜もふけた」「暮れもおしつまった」「今や春もたけなわ」「ついに夏休み[戦争]も終わった」

So the も expresses 軽い詠嘆, light exclamation/admiration.

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もう終わりだ is equivalent to もう終わりです, which means, it's (already?) over.

You're right. だ/です is a copula. です is the polite version of だ.

もう has several meanings such as "already" "now" "soon" "shortly" "anymore" etc., and I think the もう in your example can be "now" "already" or "soon":

夏休みももう終わりだ。
Summer vacation is over now.
Summer vacation is already over / has already ended / is already coming to an end.

Summer vacation will be over soon.