A site called God Discussion has helpfully compiled several ways various religious groups celebrate Valentine’s day, or not.
The Restored Church of God (an offshoot of the former Worldwide Church of God–now named Grace Communion International) thinks nobody should be celebrating it because it’s a pagan holiday
It turns out the original pagan holiday was a Roman sex festival called Lupricalia and sounds a lot more fun. I’ll nick some details from Wikipedia:
Lupercalia was an ancient pastoral festival, observed on February 13 through 15 to avert evil spirits and purify the city, releasing health and fertility. Lupercalia subsumed Februa, an earlier-origin spring cleansing ritual held on the same date, which gives the month of February its name.
Oh, the things I don’t know. I must have wondered where the word February came from at some point but it never occurred to me to look it up. Interesting. Anyway, according to Wikipedia, celebration of this particular festival hung in until the late fifth century for some Christians of Rome but Christian piety finally usurped this event once Pope Gelasius I succeeded in abolishing it in 496.
People still wanted something fun to do in February, being creatures of habit and tradition, so the festival was soon replaced with the Feast of Valentine. And, as a nod to those men who once had a Lovers Lottery to pick which single girl they’d party with, the Church ran a lottery of Saints. According to this, whatever saint got picked was the one they’d try to emulate all year.