Tsuyu (Rainy Season)

"Tsuyu" or "Baiu" is a type of rainy season. It's marked by a long period of overcast weather and rain that isn't particularly heavy.
The word uses the Chinese characters for "plum" and "rain" and there are few possible explanations for why this is. One explanation is that it's during this time that plums ripen. Another is that the humidity at this time makes it easy for mold to grow, so people called it "baiu" with the characters for "mold" and "rain," and then later changed it to "plum" and "rain," which has the same pronunciation.