Every month is a dead month. If you believe all you read about this subject, you can find some "expert" who will give you a reason not to send out a query during any one of the twelve months. It's all nonsense.
Whether your query gets acted on or not, it's still ahead of those who wait, and publishing is often first come, first served. I've never had an agent who didn't send submissions out during every month of the year, and I've sold things to publishers every month of the year except, I think, March. In fact, August has been the second most productive month I've had for sales. Who knows, maybe this is because no one else is sending editors anything to buy during August.
I don't know an editor who doesn't keep is solid touch when on vacation. They read e-mails, they read submissions, they make phone calls, and do everything else they possibly can.
Seriously, look around and check out all the months you aren't supposed to send out queries or other submissions. If you follow all the advice written by those in the industry, you'll only query on the sixth Thursday of the thirteenth month of the year.