There’s no time as such. Apple don’t make any changes. If certain app developers want to sell their apps on discount they can do any time.
You can use http://appshopper.com to track price movements.
Add the apps that you're interested in to your wishlist and you should receive an email notification when they drop in price, although I must admit it's a while since I received an email from them.
Never heard of the App Store having a big sale.