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)