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 corresponds roughly to June in the modern calendar.
Modern 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.
SEASONAL WORD (季語):
五月晴れ is a traditional seasonal word (kigo) for early summer in haiku.