Hottest Day for each year for each station.
This requires a matrix of years as rows, stations as columns and cells as the days that reached the hottest temps. The spreadsheet for that can be downloaded from here.
This is the plot of how many of those hottest days land in each month for each year. If there is any migration of the hottest days moving into other months, the loss of one month should show up as a gain in another. But that is not what is happening. Most of the heat loss, the drop in summer temps, is occurring almost exclusively in July. This means the summers are forming more of a plateau than a crest since 1920.