Learn Hardcore Japanese: kyuuzitu de mo, nihongo wo benkyousimasu.

休日 means “holiday” or “day off.”
The particles で (indicating context/time “on”) + も (“even”) combine into でも, giving “even on.”
So 休日でも means “even on holidays.”

Why do we need で here? Could we just say 休日も?

休日も literally means “holidays too,” but it doesn’t indicate “in the context of holidays.”
で marks the time/place of an action (“on holidays”), and adding も then makes it “even in that situation.”
Thus 休日でも emphasizes “even when it’s a holiday,” not just “holidays also.”

What nuance does this sentence convey compared to 休日は日本語を勉強します?

Using 休日は (topic marker は) simply states “As for holidays, I study Japanese.”
Using 休日でも adds a contrastive nuance: “Even on my days off, I still study Japanese.”

Why is the subject omitted? Who is doing the studying?

In Japanese, subjects are often dropped when they’re clear from context. Here, the speaker (“I”) is understood. So the full idea is “I study Japanese even on holidays.”

What role does を play in 日本語を勉強します?

The particle を marks 日本語 as the direct object of the verb 勉強します (“to study”). In other words, “study Japanese.”

Why is there a comma after でも?

Japanese uses the comma (読点) to indicate a slight pause or separation of clauses. Here it highlights the contrastive phrase 休日でも before the main action.

Are there other ways to say “study” besides 勉強します?

Yes. 勉強します is the polite present/future form. Other forms include:

Dictionary form: 勉強する
Casual present: 勉強する
Past polite: 勉強しました
Negative polite: 勉強しません
Te-form: 勉強して
Each form adjusts politeness, tense, or grammar structure.