There are many pages around the internet which say something like this:
The feast of St. Valentine of February 14 was first established in 496 by Pope Gelasius I, who included Valentine among all those “… whose names are justly reverenced among men, but whose acts are known only to God.”
But the quotation is never referenced. Often it is not applied to St Valentine, but to St George instead. Sometimes people mention the “Canon of Pope Gelasius”.
In fact the wording is an old – at least 19th century – and loose translation of a passage from the Decretum Gelasianum. Latin:
item gesta sanctorum martyrum, quae multiplicibus tormentorum cruciatibus et mirabilibus confessionum triumphis inradiant. quis catholicorum dubitet maiora eos in agonibus fuisse perpessos nec suis viribus sed dei gratia et adiutorio universa tolerasse? sed ideo secundum antiquam consuetudinem singulari cautela in sancta Romana ecclesia non leguntur, quia et eorum qui conscripsere nomina penitus ignorantur et ab infidelibus et idiotis superflua aut minus apta quam rei ordo fuerit esses putantur; sicut cuiusdam Cyrici et Iulittae, sicut Georgii aliorumque eiusmodi passiones quae ab hereticis perhibentur conpositae. propter quod, ut dictum est, ne vel levis subsannandi oriretur occasio, in sancta Romana ecclesia non leguntur. nos tamen cum praedicta ecclesia omnes martyres et eorum gloriosos agones, qui deo magis quam hominibus noti sunt, omni devotione veneramur;
Rendered in B. Neil & P. Allen, Letters of Gelasius I (492-496), Brepols (2014), p.160:
Likewise the deeds of the holy martyrs who beam forth among their multiple and excruciating torments the amazing triumphs of their confessions. What catholic could doubt that they suffered those things and more in their struggles and did not bear all these things by their own strength but by the grace and help of God? But according to an ancient custom, by an unparalleled security measure in the church of Rome both those deeds whose authors’ names are totally unknown and are thought to be written by unbelievers or private persons, being unnecessary or less appropriate than the order of the matter was, are not read: like those of a certain Quiricius and Julitta, like those of George, and passions of others of this kind, compositions produced by the heretics. Therefore, these are not read in the holy church of Rome, as has been said, to prevent even a slight chance of derision from arising. However, for our part, we – together with the aforesaid church – reverence with all devotedness all the martyrs and their glorious struggles, which are known better to God than to human beings. Likewise we accept with all honour the lives of the Fathers, Paul, Antony, Hilarion, and all the hermits, those at least which the most blessed Jerome wrote.48
The reference to St George is genuine – indeed a 5th century “Life” of St George exists, which is indeed rather dreadful and probably heretical. But this has nothing to do with St Valentine.