ne'er cast a clout til May be out
Definition from Wiktionary, the free dictionary
May here refers to the common name of the hawthorn bush, whose blossom indicates the arrival of summer, or warmer weather.
Note: this definition contradicts the example below
- (regional) Advice not to change from winter clothes to summer clothes until June, as there is often a sudden cold snap in May.
advice not to change from winter clothes