Anything holiday related, right after that holiday. Want some cheap candy? Wait until Halloween is over, for example. We theme so much crap that doesn’t really need the theme, then stores need to get rid of it to make room for the next things, so it’s usually got huge discounts.

Best time to buy graphics cards - when crypto values are down. Reason: mining is worth less.

Best time to buy propane - the summer, when there’s usually a glut of this refining by-product due to low demand.

Best time to buy meat - at the onset of a drought. Ranchers need to downsize to keep down expenses.

Best time to buy Tesla stock - a day or two after Elon opens his mouth (be sure to sell before he opens his mouth again!)

This list might not be globally applicable - for example, I believe the best discounts for bicycles in Sweden are usually found in the post-christmas sales.