Learn Hardcore Japanese: kyonen ha siawasena hi ga ookatta desu.

In Japanese, は is the topic marker, not the subject or object marker. By saying 去年は, you’re setting “last year” as the topic of your sentence (“As for last year…”). If you used が, you’d be marking “last year” as the grammatical subject, which would sound odd here. を is only for direct objects, and “last year” isn’t something being acted upon.

Why is 幸せな used instead of just 幸せ before 日?

幸せ is a な-adjective (also treated as a noun), so to modify another noun (日), you must insert な:

幸せな日 = “happy days.”
Without な, you can’t directly attach 幸せ to 日.

What role does 日 play here?

日 means “day” or “days,” and in plural sense it refers to “days” in general. Combined with 多かった, we get “(there) were many days.”

Why is there a が after 日?

Because が marks the subject of the predicate 多かった (“were many”). Here, 日 is what is “many,” so it takes が.

Why is the adjective in the past tense (多かったです) instead of present (多いです)?

The action/state refers to 去年 (last year), which is in the past.

Present: 日が多いです = “there are many days” (now)
Past: 日が多かったです = “there were many days” (last year)

What is the function of です after 多かった?

です adds politeness. Adjectives in Japanese can stand alone (多かった), but adding です (多かったです) makes the sentence more polite, as in casual vs. polite speech.

Could I use たくさん instead of 多かった?

Yes, but the structure changes. たくさん is an adverb or noun meaning “a lot.” You’d say:

去年は幸せな日がたくさんありました。
Here, ありました (“there was/there were”) pairs with たくさん. In contrast, 多かった directly describes 日 as “many.”

How would you say this in casual (plain) Japanese?

Drop です to make it plain:

去年は幸せな日が多かった。
You could also add よ at the end for emphasis: 去年は幸せな日が多かったよ。