Should we use comma before "when" or not? Is it a restrictive or non-restrictive clause?

Question

Should we use a comma before "when" or not? I mean is it a restrictive or non-restrictive clause?

• They generally take a short vacation in January, when there is very little business in their store.
• They generally take a short vacation in January when there is very little business in their store.

I think the second clause should be a restrictive one. Am I right?

Answer 1

Both sentences are correct, but have different meanings because of the restrictive/non-restrictive difference.

The first sentence means they take a holiday in January, presumably every year, because business is slow.

This is a non-restrictive sentence, so "when there is very little business in their store" is extra information, and doesn't define "January".

The second sentence means they take a holiday at the specific time in January when business is slow. This implies there's one time in January when business is slow, and the rest of the month business isn't slow.

The intended meaning is almost certainly the first, so there should be a comma.

Answer 2

Both are correct, though they have different meanings.

In the first sentence, you are not restricting "January". Thus, January is always when there is very little business in their store.

In the second sentence, you are restricting it. That could mean that

• only in some Januaries is there very little business in their store or
• they take a vaction only during the portion of January in which there is very little business in their store.