February 14, the global day of romantic love, named for a martyred saint nobody can identify.
Documented
The Chaucer paper trail
Jack Oruch's survey of every Valentine reference before 1382 found no link to love or mating anywhere. Chaucer's Parlement of Foules, written around 1382 for Richard II's engagement to Anne of Bohemia, is the first: birds choosing mates on Seynt Valentynes day. Oruch's verdict was blunt: Chaucer was the original mythmaker in this instance.
Assessment: Documented: the poem exists, the survey exists, and nothing earlier connects the saint to romance.
Stories
The Lupercalia takeover
The popular story says Pope Gelasius I abolished the Roman fertility festival Lupercalia in 494 and replaced it with St. Valentine's Day. Gelasius did write a letter against Lupercalia, but historian Bruce Forbes and others note the letter never mentions Valentine, and no source ties the two feasts for another thousand years.
Assessment: Stories: the ban is real, the swap is not in any record.
Interesting claims
Which Valentine
The early church honored at least two different martyrs named Valentine, both reportedly executed on February 14 in different years, plus a third in Africa. In 1969 the Catholic Church removed the feast from its General Roman Calendar because so little was known about him, or them.
Assessment: Interesting claims: the day's patron might be two men fused into one, and the church itself shrugged.
Sources and further reading
Jack B. Oruch, 'St. Valentine, Chaucer, and Spring in February', Speculum 56:3 (1981), the survey finding no love link before Chaucer
Geoffrey Chaucer, The Parlement of Foules (c. 1382), lines 309-310, written around Richard II's engagement to Anne of Bohemia
Bruce David Forbes, America's Favorite Holidays (2015): no evidence connecting Valentine's Day to Lupercalia
Pope Gelasius I, letter to Senator Andromachus (494), against Lupercalia, with no mention of Valentine
Acta Sanctorum and the 1969 revision of the General Roman Calendar removing the feast
Worth asking
If one court poet invented romantic Valentine's Day to flatter a king's engagement, which of your other traditions started as someone's paid work?