or card sent to a sweetheart" first recorded 1824. The romantic association of the day is said to be from it being around the time when birds choose their mates.
"For this was on seynt Volantynys day
Whan euery bryd cometh there to chese his make."
[Chaucer, "Parlement of Foules," c.1381]
Probably the date was the informal first day of spring in whatever Fr. 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.