The best time to contact an agent, hands down, is when your manuscript is as perfect as you can possibly get it. I know, I know, this isn't the answer you wanted, but it's the truth. In my opinion, far too many aspiring authors send out a manuscript before it is ready...they're just too antsy and want instant gratification. I'm not just talking about typos, though they're those too. I'm talking about first or second drafts that simply aren't major league ready, and sending out before you are in top form can really impede you in the process. Once an agent passes, he or she is unlikely to take a second look, even if you've majorly revised. (Yes, very occasionally he or she will, but why take that risk?)
So how do you know if you're in game day form? That's the million dollar question, of course. As I've said here, find trusted readers, listen with open ears and no ego to criticism, go through and delete, delete, delete extra exposition and scenes that don't propel the plot forward. Most importantly, I think, is to sit on it for a while. A few extra weeks won't kill you. Take a step away from it and then reread it and see if you still think it's just as genius as before. Try to remember that agents see SO MUCH STUFF that if yours isn't the cream of the crop, they'll likely pass without a second thought, and you'll be shooting yourself in the foot by sending it before it's at its very best.
Now, to answer your question, yes, August is a slow time in the industry. I'm not sure that I'd submit then, nor would I submit over the December holidays BUT, I'm also not sure that I wouldn't. (Gee, helpful, right?) Plenty of agents are still working in August (though yes, the last week is really, really dead, so I'd skip those already clogged inboxes), and things might be slow enough that you might grab their attention. I'm certain that there are writers, perhaps even who read this blog (chime in!), who garnered attention during this so-called slow time.
So again, the best thing I can tell you is sure, to perhaps be wary during August, but just to make sure that your ms is in kick-ass shape and send it in THEN. And good luck!
Readers, what say you? Better or worse times for submission?