Just as Europe closes up shop for the month of August, so too does the publishing industry go on a long hiatus. With the Labor Day holiday approaching, the upcoming week will be a peak period of agent vacations.
Is it a good time to send a query? Something posted via snail-mail will arrive and sit on a desk, untouched, unless the agent you're submitting to is one with an assistant to handle the paper work. E-mail queries may be met with an automatic response that says the agent is out of the office until early September but that query will get read then, don't you worry. I'm of a mind to do the worrying, in spite of the promise, because it's too easy to overlook an e-mail. Especially when the agent comes back to an inbox with hundreds of messages.
Go on and query, even though you know there's no one at home. Your letter will sit on the desk a bit longer, but if it manages to land at the top of the pile, when the agent is refreshed and ready to find your little gem, what could be better? So you have to wait longer. If you've been at this querying process for a time, you're a master of waiting.
And do you have your short story submissions ready to go? Literary journals start opening up in August and almost all are reading in September. Wouldn't some credentials look lovely on your author bio paragraph in your query? Better than a simple "Thank you for your consideration", isn't it?