Prior to 60 years ago, 100 degree weather in the US was quite common during September. During the early 1950’s almost one fourth of the US would reach 100 degrees sometime during September. During September 1939, 40% of US stations reached 100F during September. But since 1955 the US has seen little September hot weather, with 100 degree weather mainly confined to the desert southwest.
Wait till I post the heatwave data of September 29, 1953. It will blow you away.