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The proverb ne’er cast a clout till May is out, and its variants, mean: do not to discard winter clothes too soon.

In this proverb:
– the noun clout is applied to any piece of clothing—as explained, for example, in quotation 6 below;
– the original reference is to the end of the month of May and not (as is sometimes thought) to the appearance of spring blossom—as explained, for example, in quotation 3 below.

These are, in chronological order, the earliest occurrences of the proverb ne’er cast a clout till May is out and variants that I have found [cf. note 1]:

1-: From Memoires of the Lives, Actions, Sufferings & Deaths of those Noble, Reverend, and Excellent Personages, that suffered by Death, Sequestration, Decimation, Or otherwise, for the Protestant Religion, and the great Principle thereof, Allegiance to their Soveraigne, in our late Intestine Wars, from the Year 1637, to the Year 1660. and from thence continued to 1666. With the Life and Martyrdom of King Charles I (London: Printed for Samuel Speed [&c.], 1668), by the Welsh biographer David Lloyd (1635-1692) [page 650]—the following is about “the Right Honorable Henry Cary, Earl of Munmouth [sic]” and “his Eldest Son”:

The first of these Honorable drank no Wine till he was thirty years of Age, saying, it preyed upon the natural heat, and that vinum est Lac senum bis puerorum [cf. note 2]: the other enjoyed health best in unhealthy places, whence he observed that the best Airs for a man, are those that are contrary to his temper, the moist to the dry and consanguine, and the dry to the moist and phlegmatick, and the best Diets to those that correct the Air; and the best method a care of not going from one extream into another, urging often that saying,
Till May be out,
Leave not off a Clout.

2-: From Gnomologia: Adagies and Proverbs; Wise Sentences and Witty Sayings, Ancient and Modern, Foreign and British (London: Printed for B. Barker [&c.], 1732), by the British physician Thomas Fuller (1654-1734) [page 276]:

6191 May come early, come late,
’Twill make the Cow to quake.
6192 An hot May, as I have heard,
Maketh a fat Church-yard.
6193 Leave not off a Clout,
Till May be out.
6194 A May-Flood
Never did good.

3-: From A Familiar Medical Survey of Liverpool. Addressed to the Inhabitants at Large. Containing Observations on the Situation of the Town [&c.] (Liverpool: Printed by H. Hodgson, 1784), by William Moss, surgeon at Liverpool [Of the necessity of a strict attention to the Cloathing, page 93]:

A few warm days in April will, with many, remove the warm winter’s cloathing, and almost as certainly supply a cold: and colds, thus caught, frequently lead to the most fatal terminations. The weather rarely or never becomes regularly warm ’till the month of June; so that the valetudinarian, who is affected by cold, should be cautious in changing his winter’s dress before that time; and would do well to remember the following old familiar adage;
Cast not a clout,
’Till May be out.

4-: From Chambers’s Information for the People. New and Improved Edition (Edinburgh: Published by William and Robert Chambers, 1842), edited by William and Robert Chambers [volume 1, Proverbs and Old Sayings: Scots Proverbs, page 793, column 2]:

Cast not a clout till May be out.

5-: From Beauchamp; Or, The Error (Leipzig: Bernhard Tauchnitz, 1846), by the British novelist and historical writer George Payne Rainsford James (1799-1860) [chapter 1, page 3]:

It was the month of May—now if poets and romance-writers, as we have before said, have done injustice, or more than justice to spring, as a whole, never were two poor months so scandalously overpraised as April and May. The good old Scotch poet declares that in April,
Primroses paint the sweet plain,
And summer returning rejoices the swain,
but rarely, oh, how rarely, do we ever see primroses busy at such artistical work; and as for summer, if he is returning at all, it is like a boy going back to school, and lingering sadly by the way. Such, at least, is the case now-a-days, and if the advice of another old poet, who tells us,
Stir not a clout,
Till May be out,
would seem to prove that in ancient times, as well as at present, May was by no means so genial a month, as it has pleased certain personages to represent it.

6-: From Popular Year Book. May, published in Sharpe’s London Magazine: A Journal of Entertainment and Instruction for General Reading (London, England) of Saturday 2^nd May 1846 [page 12, column 1]:

There is another rhyme, which is not over flattering to the favourite month of the poets:
“Till May be out
Change na a clout.”
That is, thin not your winter clothing till the end of May.

Notes:

1 There is some confusion in the Oxford English Dictionary (current online edition), s.v. clout, noun 1, II.4.b. The proverb appears as follows, at the end of a quotation from A Chronicle at large and meere History of the affayres of Englande and Kinges of the same [&c.] (London: Printed by Henry Denham, 1569), by the English printer and historian Richard Grafton (circa 1511-1573):

The..Peysauntes spoyled the deade Carcasses, leauing them neyther shyrt nor clowte. Old Proverb, Till May be out Ne’er cast a clout.

But, in fact, the proverb does not occur in Richard Grafton’s Chronicle. It seems therefore that “Old Proverb, Till May be out Ne’er cast a clout” was an editor’s note in the first edition of the Oxford English Dictionary (volume 2. C., 1893 – s.v. clout, noun 1, II.4.b, page 529, column 1).

2 The Latin phrase vinum est Lac senum bis puerorum translates as: wine is the milk of old men [who are] twice children.