During the summertime it usually starts to get hot outside around 12:00 PM although it may get hot sooner depending on the location.
Usually the coolest times during summertime are the early morning hours before 11:00 AM and the hottest parts of summer are between 12:00 PM to 5:00 PM
The hottest time of day in summertime is around 3:00 p.m.
Heat continues building up after noon, when the sun is highest in the sky, as long as more heat is arriving at the earth than leaving.
By 3:00 p.m. or so, the sun is low enough in the sky for outgoing heat to be greater than incoming.
If you want to avoid the heat you should go out early in the morning or in the evening hours as the sun goes down.
If you need to exercise then you need to avoid the heat when possible as you can easily get heat stroke and die as a result.
If you don't have to be outside during the hottest time of day it's best to avoid being out in the heat and stay in air conditioning as much as possible.
However if you do need to be outside you should take frequent breaks, drink plenty of water, cool off or if having fun go to a splash pad, swim in a swimming pool or run around in a sprinkler.