The Japanese word for the rainy season, tsuyu or baiu, is written as "plum rain" in kanji. While there are various theories as to the word's origin, a popular one holds that it's because June is plum (ume) season in Japan.
It's when the annual crop of plums are picked, umeshu (plum wine) is concocted and the pickling of umeboshi, the salty-sour preserved plum, is started.
As well as being easy to make — the preservation process takes approximately one month — umeboshi are an excellent ingredient, and cooks can expect mouthwatering results with both store-bought and homemade umeboshi.