He encouraged the picking of a "heavenly" Valentine to whom you should give a spiritual bouquet.
St. Valentine’s Day has a long and rich history, and the exchange of valentines dates back many centuries. For example, even St. Francis de Sales, who lived in the 17th century, mentions this custom.
However, he encourages an alternative where a saint is picked for someone and a card is given to them with the name of the saint they should try to imitate.
In a letter to a religious sister, St. Francis de Sales explains what kind of bouquet should be picked for this type of “Valentine.”
You ask me, my dear daughter, what bouquet you can give to your Valentine. It should be made of some little acts of virtue which you should practice expressly for the sake of this heavenly Valentine; and at the end of the morning’s meditation you shall offer it to him that he may consecrate it to your dear Beloved. You can also sometimes gather some from the garden of Olives, or from the mount of Calvary —I mean those bouquets of myrrh of your St. Bernard—and beg your heavenly Valentine to receive them from your heart, and to praise God for them, which is as if he spread abroad their perfume; for you can neither smell his divine flowers worthily enough, nor highly enough extol their sweetness.
St. Francis de Sales sought to encourage a virtuous life with this unique Valentine tradition, calling to mind that the secular holiday was originally centered on a canonized saint.
He continues in his letter, pointing out the Valentine that the sister had picked, as well as the gifts she can ask to receive from her heavenly friend.
Again you can ask him, this dear Valentine, that he also would take this bouquet and let you smell it from his hand, and also that he would give you some other in exchange; that he would give you scented gloves, covering your hands with works of charity and humility, and bracelets of coral and chains of pearls. In such way should you have loving tendernesses with these blessed knights of the King of Glory. I think it was St. Thomas Aquinas that you drew for the month, the greatest Doctor that ever was; he was a virgin, and the sweetest humblest soul that could be conceived.
The good news is that this practice could be done by anyone, whether you are married, or single, young or old. Above all, we should strive to love each other with a Christ-like love, looking to the saints for inspiration.