They sometimes happen randomly. Otherwise, Labor Day. Start checking a week beforehand.

I think it's good that you missed the July 4th sales if you are not planning to install until mid-Sept. Please note that the warranties on appliances start when the appliances are delivered, not when they are installed. If you bought on July 4 weekend, and didn't install until mid-Sept. (and those dates tend to change as we all know), you would lose 2-1/2 months of your warranty.

As already said, the next holiday to get sales is the Labor Day weekend, but frankly, you can get a discount any time with a little bargaining at Sears, Best Buy, etc. or even the online stores (unless you are buying Subzero/Wolf, Miele or Gaggenau which don't discount). I bought a microwave recently online, and called the website number to buy rather than make the purchase online. I said to the guy, "can you do a little better on the price?", and he took off $20. While $20 isn't a huge amount, the MW wasn't very expensive to begin with, and hey, I could always use an extra 20 bucks. So don't worry about sales on certain dates. You can get discounts other times as well.

Also if you are looking at Kitchenaid, make sure to go to the Kitchenaid website - they often offer discount coupons. A friend of mine bought a Kitchenaid dishwasher, filled out the Kitchenaid form she found online, and they sent her a $50 Master Card. Likely some of the other brands do those kinds of promotions as well so check out the websites.

Two best times to buy appliances,deepest discounts, are 4th of July and Black Friday with Black Friday the best. Usually starts around Veteran's Day.