Throughout the year there are these things called "Holidays." Let's see: the Fourth of July, Valentine's Day, St. Patrick's Day, Halloween, and on and on it goes. Yep, there are holidays all over the place.
A perk to writing any series, not just a cozy mystery series, I would assume, would be the seasonality of stories. For example, the book I'm reading now, Santa Clawed, is set around Christmas. So author Rita Mae Brown can get into the holiday spirit a little more than an author who is writing a stand-alone kinda thing. She can utilize characters that she's already attached to and create a story around them that still lets her celebrate whatever time of year she's writing about.
I know--just because a story is about Christmas doesn't mean it wasn't written in July. (Were there enough double negatives in that last sentence?) It also doesn't mean that, as an author trying to build a 'stable' of novels, that if you're writing around Valentine's Day you should ignore the possibilities of that romantic-al-est time of year. Then whenever the publisher needs something timely--VOILA! You have produced.
And here is Rita Mae Brown her-own-self talking about Santa Clawed: