The phrase "n'er cast a clout till May be out" is usually interpreted as "don't cast off your winter clothes (clout) till the month of May is over". (I always thought that a bit extreme, even if the weather was colder in olden times).
However a wise woman of my acquaintance tells me that, in this context, 'may' means the may-thorn, a variant on the hawthorn. Ah.
Well, the hot weather of recent days has coincided with the blossoming of the may-thorn in the fields.
So - now it makes sense.