Its entirely at the individual developers discretion.
Of course, but as an example, Black Friday was a time when a lot of software developers, notably some big companies, put their apps on sale. I'm asking because I only recently got my iPod Touch and am curious to know were there any other times besides Black Friday during when there were big sales.
Because it is a digital good not limited by supply, there is no universal rhyme or reason for when a developer will put an app for sale. In general, it probably has a lot more to do with how well the app is doing on the app store, then any calendar date.
Its entirely at the individual developers discretion.
My all time favorite so far's App Shopper (won't post the link as I've been censured on other forums on other sites before for that, lol) .