Ne'er cast a clout till May be out. Meaning?

Today across southern England, it was one of those glorious May mornings of which the poets wrote. The darling buds in bloom, the scent of the blossom hanging like nectar in the air, and the sun up in a clear sky, temperature in the mid-twenties.

Yet I took a pullover with me when I went out. Why? Because I have never forgotten the old English proverb taught to me by my grandmother 'ne'er cast a clout, till May be out'.

The earliest written version is that of Dr Thomas Fuller in 1732. But the argument has never been settled. Does it refer to the month of May? Should we not leave off an item of clothing until June arrives, or does it refer to the 'may flower' on the hawthorn which blooms along the 200,000 miles of English hedgerow from late April?

The excellent coverage of this question at The Phrase Finder points out an earlier (than 1732) source that conveys the gist of the saying in a translated Spanish proverb:

Captain John Stevens's work, 'A New Spanish and English Dictionary', published in London in 1706, translates a Spanish proverb, as "Do not leave off your Coat till May be past".

But then the author of this article observes:

Those rhymes may well have originated in England and migrated across the Channel. It is difficult to understand why the Spanish would coin such a proverb, which would seem a little cautious for that part of the world - the average temperature in Seville in May is 20°C.

The Phrase Finder's author ultimately votes firmly in favor of the end-of-the-fifth-month-of-the-year interpretation of the saying over the hawthorns-past-their-bloom interpretation:

All in all, although the May blossom interpretation seems appealing, the 'May' in this proverb is the month of May.

Jennifer Speake, Oxford Dictionary of Proverbs, Fifth Edition (2008), cites both Stevens and Fuller in her brief coverage of the saying—and then offers this interesting quotation from Robert Graves, The White Goddess (1948):

In ancient Greece, as in Britain, this [May] was the month in which people went about in old clothes—a custom referred to in the proverb 'Ne'er cast a clout err May be out,' meaning 'do not put on new clothes until the unlucky month is over.'

So the Graves theory seems to be that the May referred to is indeed the month of May, but that the point of not casting off a clout during May has to do with an old English (and ancient Greek) superstition about not prematurely donning new clothing during the month of May.

Followup: Thanks to the excerpt that jlovegren provides from a different edition of The Oxford Dictionary of Proverbs (and to Mari-Lou A's asking about it), we can see that an earlier version of the Spanish form of the proverb has existed at least since 1627—but instead of saying "Until May has passed..." it says "Until May..." That suffices to get the citizens of Seville out of their overcoats on May 1, and it gives Spain a 105-year head start in documented existence of the proverb over the first native English occurrence. It begins to look to me as though Spain has the stronger claim.

I'll go ahead and post the entry from Oxford Dictionary of Proverbs

[Sp. c. 1627 CORREAS Vocabulario (1906) 480 Hasta Mayo no te quites el sayo. 1706 STEVENS S.V. Mayo, Hasta passado Mayo no te quites el sayo, Do no tleave off your Coat till May be past. Cf. also S.V. Abril, En Abril no quites fil: In April do not take off a Thread. Cf. also Sayo, Guarda el sayo, para Mayo, Keep your great Coat for May, that is, do not part with it before May. 1732 FULLER no. 6193 Leave not off a Clout, Till May be out. 1832 HENDERSON 154.

According to the following source, your second assumption seems to be the probable origin of the saying:

The earliest published version version of the rhyme can be found in Dr. Thomas Fuller’s “Gnomologia” published in 1732.

Since at least the early 15th century ‘clout’ has been used to mean a fragment of cloth or clothing and was spelled as clowt, clowte, cloot, or clute. It’s here that the saying took on two meanings rather than just the original. The new meaning was a reminder not to be too quick to shuck the warmer winter clothes before cooler days during the month of May were most likely over.

That being said, English farm-workers working the fields in their winter clothes throughout the month of May could suffer from heat exhaustion if they kept all their winter layers on until the end of May! The flowering of the hawthorne (May) tree was a more reliable guide to the state of the weather.

This means that the original meaning goes back even further than the 15th century and indeed, it can be traced back to the 12th century. During Medieval times in Brittany, a man proposed to his beloved by leaving a hawthorne (also known as a Mayflower) branch at the door of his beloved on the first of May. By leaving the branch at the door she accepted his proposal.

Traditionally, it was taboo to bring hawthorne into the house in Medieval England because it was feared it would bring death with it. This is because the hawthorne blossom has a distinctive fragrance and in medieval times, the blossom was said to carry the ‘stench of death’. (This is due to the trimethylene that the flowers give off as they deteriorate.)

The exception to that rule was during May-Day celebrations (for one day only) when it was permitted to bring flowers into the house for decoration. No marriages were allowed during the month of May and it was considered unlucky to marry in the hawthorne month since most people during Medieval times rarely bathed, June was usually one of the months in which most people had baths. The exception to the rule, of course, would be those who lived in castles.

(emphasis mine)