It might help to separate the two things you are thinking about
Maximum heat gained in one day
Vs
Maximum temperature stored
Each day either adds to the stored heat or let’s more heat escape (cooling). The solstice is just the day we can add the most to the storage in one day. But we will add some tomorrow and the next day until we hit the max and then it begins to cool…
Note: this is a vast oversimplification but address the root concept.
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