1.
clear weather in May; fine day during the rainy season
originally referred to brief clear spells during the rainy season (the fifth month in the lunar calendar), now commonly used for pleasant clear weather in May
Compounds
五月 (fifth month/May in lunar calendar) + 晴れ (clear weather)
Etymology
Originally referred to clear spells during the rainy season (梅雨), which fell in the fifth month of the lunar calendar. The lunar fifth month falls roughly between late May and early July in the modern calendar.
Usage
Today, 五月晴れ is commonly used to describe pleasant, clear weather in May (modern calendar). Weather forecasters sometimes note this shift from the original meaning.
Cultural note
五月晴れ is a traditional seasonal word (kigo) for early summer in haiku.