noun
fine weather during the rainy season
Refers to clear, sunny weather that appears during the 梅雨 (rainy season), typically in June. Often used poetically or in weather forecasts.
今日は五月晴れで、気持ちのいい一日になりそうだ。
Today is a fine day during the rainy season, and it looks like it will be a pleasant day.
梅雨の合間の五月晴れを狙って、洗濯をした。
Taking advantage of the sunny break in the rainy season, I did the laundry.
五月 (さつき) is an old name for the fifth month of the lunar calendar, corresponding to the modern rainy season. 晴れ (ばれ) means clear weather. The compound refers to sunny spells during the rainy season.