Yamaksa Dictionary

Rainy season:
Southern Japans rainy season starst at the end of May and ends mid July. The Hakata-Gion-Yamakasa festival is also supposed to mark the end of the rainy period, which it often does with astounding accuracy.

As can be see on this site [click here], the average weather of the last 30 years has followed pretty much exactly the aforementioned pattern. Mid July usually constitutes the end of the rainy season.