It's a Greek Saying
It appears several sources support the claim that the proverb, one swallow does not a summer make, has its origin in Ancient Greece.
From Nicomachean Ethics, Aristotle (384 BCE - 322 BCE) uses the term ἕαρ which is Greek for ‘springtide’ and ‘spring’: "One swallow does not make a spring, nor does one sunny day; similarly, one day or a short time does not make a man blessed and happy"
An alternative version: “Moreover, to be happy takes a complete lifetime; for one swallow does not make spring, nor does one fine day; and similarly one day or a brief period of happiness does not make a man supremely blessed and happy.”
In opposition, the website BookBrowse argues that the poetic proverb probably pre-dates Aristotle:
Many fables have been written based on the proverb including one attributed to Aesop. However, consider that Aesop lived around 600 BC (or at least the legend that became Aesop appears to have originated around this time as there's some uncertainty whether Aesop actually existed), it would seem that the proverb dates back at least a few hundred years before Aristotle.
In Aesop's fable, a young man sees a swallow on a warm winter day. Thinking that winter is over, he sells off his woolen coat, and with the money he's made he goes to the bar and drinks. Unfortunately, in the days that follow the temperature drops and the young man, shivering in the cold, realizes that one swallow does not make a summer.
In its article The Young Man and the Swallow, Wikipedia tells us that the proverb was already well-known in England before Aesop's fable, translated in English, appeared in 1600.
Although the fable was translated into Latin prose during the 15th century, it was not included in European vernacular collections of the time but begins to be recorded in the 16th. Poetic versions are included in French in Les Fables d'Esope Phrygien, mises en Ryme Francoise (1542)5 and in Latin by Hieronymus Osius (1564). In England the fable does not appear in collections before the 17th century, but the proverb, in the form 'One swallow does not make a summer', is recorded a century earlier. Erasmus includes its Latin version in his Adagia and the proverb is common throughout Europe.
1539 R. TAVERNER tr. Erasmus' Adages 25 “It is not one swalowe that bryngeth in somer. It is not one good qualitie that maketh a man good”
1546 The Proverbs of John Heywood. The following snippet on p.121 is from an 1874 edition.
One swallow maketh not summer.
One swallowe prouveth not that summer is neare.
Why do British (and Spanish) swallows migrate later?
If the French and Italian proverbs une hirondelle ne fait pas le printemps and una rondine non fa primavera correctly identify spring as being the migratory period, since when do swallows migrate to the British Isles in June and July?
Well, it could be that the term spring, the season following winter, was still in its infancy when the proverb came into vogue. In Old English, the season was known as lencten "springtime, spring," while summer is a much older word. Etymonline, the online etymology dictionary par excellence, confirms:
spring season following winter, the vernal season, c. 1400, earlier springing time (late 14c.), which replaced Lent, the Old English word. […]The notion is of the "spring of the year," when plants begin to rise (as in spring of the leaf, 1520s), from the noun in its old sense of "action or time of rising or springing into existence." It was used of sunrise, the waxing of the moon, rising tides, etc.; compare 14c. spring of dai "sunrise," spring of mone "moonrise," late Old English spring "carbuncle, pustule."
Whereas summer is derived from Old English sumor and from Proto-Germanic *sumur- (cognates: Old Saxon, Old Norse, Old High German sumar, Old Frisian sumur, Middle Dutch somer, Dutch zomer, German Sommer)
I'm no poetess, but both swallow and summer begin with the letter S, and both words have two syllables, and both contain double consonants, so perhaps the two scan better in unison; especially in its 16th century variant. The “original” phrase looks more symmetrical,
one swallow maketh not summer
one swallow maketh not Lent
one swallow maketh not spring
P.S. Spanish swallows also appear to travel later compared to their French and Italian cousins:
- una golondrina no hace verano (wiktionary)
P.P.S And a special thanks to @Peter Shor who pointed out that Aristotle's original citation had the term spring.