Black Friday through Cyber Monday is always the time where you will see large discounts happening. It is usually a Cyber Week since some will start listing items before Thanksgiving and go through the following week (or to Xmas).
For an entire inventory sale; it does happen but that could be whenever the store chooses (President's Day Sale, St. Patrick's Day Sale, Look-A-Speckled-Warbler-Pooped-On-Our-Car-Today Sale, etc).
You are quite right about that. It does happen but no one knows when. Lately there has been at least 4 big sales a year from MM so maybe I will look out for that. CH had on 4th of July 2 years ago IIRC.