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)
We did that last fall, turned it on on a hot spring day and it promptly died. (Thankfully it was early enough in the season that we were able to get a reasonably priced installation for a new one at the end of June).