イタリアの守護聖人であるラテン語のヴァレンティヌスから、文字通り強さ、能力を意味する、勇ましいとの語源は、その記念日が鳥が互いの相手を選ぶ時期と重なることから、バレンタインデーを意味する語源になったと言われている。
For this was on seynt Volantynys dayProbably the date was the informal first day of spring in whatever French region invented the custom (many surviving medieval calendars reckon the start of spring on the 7th or 22nd of February). No evidence connects it with the Roman Lupercalia (an 18c. theory) or to any romantic or avian quality in either of the saints. The custom of sending special cards or letters on this date flourished in England c. 1840-1870, declined around the turn of the 20th century, and revived 1920s.
Whan euery bryd cometh there to chese his make.
[Chaucer, "Parlement of Foules," c. 1381]
To speak of the particular Customs of the English Britons, I shall begin with Valentine's Day, Feb. 14. when young Men and Maidens get their several Names writ down upon Scrolls of Paper rolled up, and lay 'em asunder, the Men drawing the Maidens Names, and these the Mens; upon which, the Men salute their chosen Valentines and present them with Gloves, &c. This Custom (which sometimes introduces a Match) is grounded upon the Instinct of Animals, which about this Time of the Year, feeling a new Heat by the approach of the Sun, begin to couple. ["The Present State of Great Britain and Ireland" London, 1723]