YSK because the first hot spell is when most people discover their AC unit is broken.
Several years ago I waited to turn on my AC. This is how I found out about one of the busiest times for repairmen and had to sweat for two weeks. Now I do a test run a couple of weeks before it heats up. Same goes for the furnace at the end of Autumn.
Edit to add: YSK line (Rule 2)
I have window units. You think I’m going to leave those in during winter? I like my heat inside my house, thank you very much.
I’m just saying it’s better for the units to run once in a while, and to test them to know they are still working. I guess you could technically run a window unit for a few min when it isn’t in the window but watch out for condensation drain.