I'm surprised that nobody's covered release days yet. App usage/download peaks on Friday through Sunday (that's based off a number of apps I work on with million+ users). If you release an app on Monday you might miss out on the boost you'd otherwise get from charting on a Thursday/Friday and having that momentum pick up through the weekend*. Apple also rotate the store promos on a Thursday (usually).
As to the seasons: it's a bit of a crap-shoot. It's been mentioned by other people that releasing during a hardware launch might not be a good idea, but on the other hand during a hardware launch there's a lot of new devices coming onto the marketplace, and owners exploring the app store for the first time.
Really, though, it's all about the content of your app. Perhaps July/August are slower months than normal, but probably not if your app is a picnic spot finder. And releasing a snow-forecast app in the summer probably isn't the best idea in the world.
*...although I'm making a western-centric assumption here, because there are some iTunes territories where the weekend doesn't fall on a Sat/Sun.