Learn Hardcore Japanese: natuyasumi ha totemo tanosii desu.

It marks 夏休み as the topic. When you see は, read it as wa—it tells you that “summer vacation” is what the sentence is about. The rest of the sentence gives information about that topic. It is not the subject marker (that role belongs to が).

Why isn’t が used instead of は?

が marks the grammatical subject, often introducing new or focused information. Here, we’re making a general statement about a known topic, so は is appropriate. Using が would shift the emphasis to the subject itself rather than the overall comment.

What role does とても play and why is it placed before 楽しい?

とても is an adverb meaning very. It modifies the adjective 楽しい by intensifying it. In Japanese, adverbs normally appear directly before the word they modify.

Why is です added after the adjective 楽しい?

Japanese i-adjectives like 楽しい can stand alone, but adding です makes the sentence polite. Here, です functions as a copula, turning “fun” into “is fun” politely.

How would you say Summer vacation was very fun (past tense)?

Convert 楽しい into its past tense form 楽しかった, then add です for politeness:
夏休みはとても楽しかったです。

Why is there no explicit subject in the sentence?

Japanese often omits the subject when it’s clear from context. Here, 夏休み is already marked as the topic with は, so there’s no need for a separate “it” or “summer vacation” as subject.

Why isn’t there a の between 夏 and 休み?

夏休み is a single compound noun meaning “summer vacation.” When two nouns form a fixed phrase, they typically join directly. You’d use の when indicating a relationship between two separate nouns (e.g. 部屋の窓).

Do you need to write spaces between words like in 夏休み　は　とても　楽しい　です?

No. Native Japanese writing doesn’t use spaces between words. The spaces here are for learners to highlight each part. In real text, you’d write:
夏休みはとても楽しいです。