In the sentence you provided, the comma rule being applied is related to the use of a coordinating conjunction in a compound sentence. Specifically, the phrase "for we have plans to visit several National parks" is an independent clause that is connected to the first independent clause, "We are excited for summer vacation."
In this case, the coordinating conjunction "for" is used to explain the reason for the excitement about summer vacation. When two independent clauses are joined by a coordinating conjunction (for, and, nor, but, or, yet, so), a comma is typically placed before the conjunction.
However, since the sentence does not include a conjunction that requires a comma (as it is not a compound sentence), there technically should not be a comma in your original sentence unless it includes an introductory element or additional clauses that warrant separation for clarity.
To clarify: "We are excited for summer vacation for we have plans to visit several National parks" can indeed be rewritten with a comma for improved readability:
"We are excited for summer vacation because we have plans to visit several National parks."
Here, using "because" as a conjunction introduces a dependent clause, making the structure clearer without needing a comma rule for conjunction.