Usage of "in" before were in a sentence from Shoe Dog

Solution 1:

After clarification from a comment under the question, Reserves is short for Army Reserves. To avoid confusion, I will add that word to the sentence.

Some Tuesday nights in the Army Reserves were set aside for classroom time.

The subject of this sentence is Tuesday nights in the Army Reserves. Or, in simplified form, just Tuesday nights.

It means that some Tuesday nights are used for classroom purposes.

Without the preposition, the construction (and meaning) of the sentence changes:

Some Tuesday nights, the Army Reserves were set aside for classroom time.

The subject is no longer Tuesday nights. Instead, it has changed to the Army Reserves. Without the preposition, some Tuesday nights becomes a dependent clause.

The implication of this sentence is that the Army Reserves, the entire organization, stops what it's doing and all personnel and resources turn to classroom time. But while that's theoretically possible, it's not what actually occurs. So, it's not describing a factual event. It's not actually nonsensical, but it's somewhat ridiculous.

The point of the original sentence is to say that classroom time is scheduled for some Tuesday nights (in the Army Reserves)—it's Tuesday nights that should be the subject of the sentence, not the Army Reserves itself.