Jehovah's Witnesses only formally celebrate the one event that Jesus commanded his followers. The memorial of his death (1 Cor. 11:23-26). They also celebrate other events which are referred to favorably in the Scriptures such as marriages, anniversaries, the birth of a baby, graduations and many other happy occasions.
But why don't Jehovah's Witnesses celebrate St. Valentine's Day? Jehovah's Witnesses take their worship very seriously and insist upon keeping their worship of God undefiled (James 1:27). Therefore, there are two main reasons why Jehovah's Witnesses do not celebrate St. Valentine's Day:
1.) Pagan ceremonies honoring pagan gods are mixed in with St. Valentine's Day's origins.
2.) The Bible makes it clear that there is to be no veneration or adoration of Saints as intercessors with God. Our approach to the Father can only be through Jesus. (John 14:6, 14; 1 Tim. 2:5)
St. Valentine's Day - It's Pagan Associations
It would be wrong to incorporate anything used for pagan worship, into our worship or related activities. Holidays, for example, are "Holy Days" and are a part of "worship", by their very name.
Notice what the publication How It Started had to say about St. Valentine's Day on pages 46-47:
"Ancient Romans placed great stress upon holidays that fell about the middle of the shortest month of the year. Dedicated to the goddess Juno, who was the wife of Jupiter, these festivals stressed love, courtship, and marriage.
"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."
And The World Book Encyclopedia, Vol. 20, 1973 ed. on page 204 says this:
"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."
Some would say that it's wrong to pay such homage to dead men, whether "Saints" or not, but certainly it's incredibly wrong to mix in pagan ceremonies honoring pagan gods in the process.
"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)."]
What is the origin of Valentine’s Day? (rs p. 176-p. 182; Watchtower Online Library)
Can the Pagan Customs of Christmas Really Be MADE Acceptable to God? (Defend Jehovah's Witnesses)
Can a Pagan Holiday Be Made Christian? (Official Website of Jehovah's Witnesses)
Should We Venerate "Saints as Intercessors" With God?
Notice what Peter's response was to a certain situation in the account of Acts 10:25, 26, JB:
"As Peter reached the house Cornelius went out to meet him, knelt at his feet and prostrated himself. But Peter helped him up. `Stand up,' he said `I am only a man after all!'"
Peter did not approve of such adoration when he was personally present. He understood what Jesus clearly stated in John 14:6, 14 that our approach to the Father can be only through Jesus and that our requests are to be made in Jesus' name:
"Jesus said: `I am the Way, the Truth and the Life. No one can come to the Father except through me. If you ask for anything in my name, I will do it.'" (JB)
1 Tim. 2:5: "For there is one God, and one mediator also between God and men, the man Christ Jesus." (NASB)
Note what Martin Luther wrote concerning the reverence of Roman Catholic Saints:
"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.
Should I Pray to Saints? (Official Website of Jehovah's Witnesses)
The Bible’s Viewpoint Is It Appropriate to Pray to “Saints”? (Official Website of Jehovah's Witnesses)