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)
YMMV But another fun AirCon fact*.
A few years ago we switched from running the air con hard at 20°C or so when we got home, having it turn off automatically late at night, running it hard in the morning and turning it off when we went out…
To…
setting it at a supposedly “high” setting of 27°C and leaving it on 24/7 (July to September)
The running cost was the same and it was very comfortable coming home to an already cool dry environment. We never felt the need to lower the temperature either. It was lovely.
*“fact”. I’m sure the condition of your home insulation, outside temperature and conditions, draftiness of windows and doors etc all play a role.