In Japan, we call the rainy season “Tsuyu”. It starts from the southernmost tip of Japan in Okinawa at the beginning of May and slowly moves north over the rest of Japan in June. Hokkaido, at Japan’s northern end, gets barely affected by the rainy season.
Tsuyu is not preferred because it is not only rainy but also humid. So, there are many kinds of useful umbrellas and various goods that you can spend comfortably in the rainy season in Japan.