It's a saints day, part of the Christian calendar, like shrove Tuesday.
"Saint Valentine was a clergyman – either a priest or a bishop – in the Roman Empire who ministered to persecuted Christians. He was sent to jail for performing weddings for couples during a time when new marriages were outlawed in ancient Rome. He was martyred and his body buried at a Christian cemetery on the Via Flaminia close to the Ponte Milvio to the north of Rome, on February 14, which has been observed as the Feast of Saint Valentine (Saint Valentine's Day) since 496 AD. "