By tradition, August is not the best time to send a query letter.
In the publishing industry, the end of summer is the time to take a vacation. There's school looming on the horizon, so it's the one last chance to get away with the kids before the school year starts up again and the running around begins anew.
Of course, you never know which agents might be hard at work and which might be sitting on the beach with a good book. You could send off a round of queries and take a chance that yours will get a look and a response. Or if you don't mind waiting longer than usual, have at it.
August is the time when literary journals begin to open their doors once again, as the university interns return. August is not the best month to submit a query, but it's the perfect time to submit short stories.
You need writing credentials these days. More and more agents make mention of such things in their submission instructions. Author bio and relevant publishing credentials? That means: "Who's thought your prose was good enough to put into print?"
Think of it as another hoop that you have to jump through on the way to getting your novel published. Oh, right, the hoop's on fire and it's a tight squeeze to make it through, but no one ever said this was an easy business to break into.