When was Saint Valentine canonized a saint?

He wasn't canonized, he was acclaimed a saint centuries before the modern system of canonization began. 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." As Gelasius implies, nothing was yet known to him about his life.