I researched the same saying some time ago
'Ne'er cast a clout till May be out' is an English proverb. The earliest citation is this version of the rhyme from Dr. Thomas Fuller, Gnomologia, 1732, although it probably existed in word-of-mouth form well before that:
"Leave not off a Clout Till May be out.
Meaning
'Cast a clout', although archaic, is straightforward. Clout is a noun, variously spelled as clowt, clowte, cloot, clute, but all meaning cloth or clothing.
So, 'ne'er cast a clout...' just means 'never discard your [warm winter] clothing...'.
The 'till May be out' part is where the doubt lies. On the face of it this would mean 'until [the month of] May is ended'.
The Hawthorn is a common tree of the English countryside, and it flowers in late April/early May. It is known as the May Tree and the blossom itself is called May. Using that allusion, 'till May is out' could mean, 'until the hawthorn is out [in bloom]'.
look at the full version of the explanation I found
http://www.phrases.org.uk/meanings/till-may-is -out.html