The Summer Solstice started Sunday. This is the third day. Tomorrow the days will start getting shorter. Oddly enough, the days will not start getting cooler. There are two analogies to explain this.
The world's axis is at a 23.5° angle. The Summer and Winter Solstices mark the extremes in it's tilt. Now, think of the swinging back and forth of the axis as the turning of the knob on the oven. At one extreme is off and at the other is... oh, say 300°F. Clearly, if the knob stayed at one extreme or the other dinner would either freeze or bake. But it takes some time for the oven to heat and cool. So the temperature has only reached 200°F or so when the knob starts coming back from 300°F. They might meet again around 230°F or so before they start cooling off. In this same manner we're usually only breaking the 90°s around here at this point in June and 100°+ in early August.
I usually hear this heating beyond the longest day referred to as Thermal Inertia. But Thermal Inertia usually refers to the second analogy. Take a metal strip and put a torch on the end you're not holding. That end will heat up. After a minute turn off the torch. Where the torch was will start to cool. However, the midpoint of the metal strip will continue to heat up as the heat moves up toward your hand. We're at the midpoint and continue to get warmer after the heat gets turned down.
Personally, I think the first example applies more than the second.