In Japan, the 梅雨 (tsuyu) rainy season typically begins in June, except for in Hokkaido.
In the traditional Japanese calendar, June is referred to as "水無月" (Minazuki).
The term "Minazuki" means "the month of water." The "無" (na) in Minazuki acts as a grammatical particle meaning "of," so it translates to "the month of water." This name originates from the period when water is added to rice fields that were previously dry. Consequently, it is also called "水月" (Suigetsu) or "水張月" (Mizuharidzuki). The rain during this season is crucial for the rice to grow, reflecting the people's hopes for a bountiful harvest.
Mountain Opening
The events of Minazuki begin with the opening of Mount Fuji on June 1st. During the Edo period, it became popular to climb Mount Fuji and watch the sunrise on this day. Mount Fuji was a sacred place of worship and not easily accessible. Common people would visit Sengen Shrines built in various locations and climb artificial mounds called Fujizuka to pay their respects.
References:
https://weathernews.jp/s/topics/202005/220185/
https://ndlsearch.ndl.go.jp/imagebank/column/minazuki