"That was appropriate since Juno, queen of heaven, was regarded as the special protector of women in `critical times of life.'....
"When Christianity became the official religion of the Roman Empire by edict of Emperor Constantine, religious leaders immediately started trying to do away with pagan festivals. Many of them had been observed for so many centuries that they resisted all frontal attacks, so there were many efforts to bring about change in a more subtle fashion. One of the most effective was a substitution of a Christian [?] observance for a pagan one, with special ceremonies held on or near the date of the long-established holiday.
"Since the martyrdom of St. Valentine was celebrated on February 14, it made sense to stress homage to the man who was said to have been stoned to death on that day in A. D. 269. For that reason the name of a man about whom no written records exist came to be attached to the day formerly dedicated to Juno." - How It Started, pp. 46-47.
"Valentine's Day comes on the feast of two different Christian martyrs named Valentine. But the customs connected with the day ... probably came from an ancient Roman festival called Lupercalia which took place every February 15. The festival honored Juno, the Roman goddess of women and marriage, and Pan, the god of nature." - The World Book Encyclopedia, p. 204, Vol. 20, 1973 ed.
Some would say that it's wrong to pay such homage to dead men, whether Catholic "Saints" or not, but certainly it's incredibly wrong to mix in pagan ceremonies honoring pagan gods in the process.
Martin Luther wrote,
"The pity is that we are so blind that we do not leave the devil alone to play his tricks in his own way; rather, we support him and multiply them. I wish people would leave the saints in peace, and not mislead humble folk. What spirit was it that gave the pope authority to canonize saints? Who tells him whether they are holy or not? .... God is just in judging us with His wrath and in allowing the devil to lead us hither and thither, to institute pilgrimages, ... to set about canonizing saints and other foolish things." - p. 458, Martin Luther, Selections From His Writings, Dr. John Dillenberger, Anchor Books, Doubleday & Company, Inc., 1961.
"What agreement is there between the temple of God and idols? .... `Therefore come out from them and be separate,' says the Lord. `Touch no unclean thing, and I will receive you ...' says the Lord Almighty." - 2 Cor. 6:16, 17, NIVSB. [NIVSB f.n.: "agreement...between the temple of God and idols. There can be no reversion to or compromise with the idolatry they have forsaken for the gospel (cf. 1 Th. 1:9)."]
For more concerning St. Valentine's Day, see:
What is the origin of Valentine’s Day? (rs p. 176-p. 182; Watchtower Online Library)
Saints (Search Results From the Watchtower Online Library)
Why Don't Jehovah's Witnesses Celebrate St. Valentine's Day? (Defend Jehovah's Witnesses)