Prior to 1955, hot summer afternoons were very common in the US, but they rarely happen any more. The vast majority of stations set their all-time temperature record in the 1930’s.
Much of the small post-1970 increase seen in the graph above, is an artifact of new stations being added. It has nothing to do with climate.
The US used to be much hotter, so NASA is tampering with data to make the heat disappear and hide the decline.