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)
You should actually be getting your annual routine maintenance as recommended by the manufacturer.
The people that installed my new HVAC this time last year called me just this morning … do you work for them?