I remember Paris hitting 97 in early June. Avignon high 90s mid-September.
Southern Spain can reach 120 degrees in August - it’s like SE Arizona in the summer.
So we tend to only visit Northern European countries in the summer, late May and early Sept.
One thing that may not be important to some but is for us is water temperature. Water temperature lags air temperature, so typically the sea is warmest in August and September. When we were in Greece for six weeks, we spent a fair bit of time on beaches and in the sea. September was perfect weather for this!