Quote from Brewer's Phrase and Fable:

Cast not a clout till May is out. An old warning not to shed winter clothing too early in the year. Clout here is a rag or patch, hence a piece of clothing. May is also another name for hawthorn, which blossoms in May. Thus some hold that the proverb means'Do not discard clothing until the hawthorn blossoms', but more likely it means 'Wait until the end of May'.