Chaucer wrote The Parliament of Fowls in 1382 to honor the first anniversary of the engagement of King Richard II to Anne of Bohemia (they were both 15 years old when they were married shortly thereafter), but it has become associated with the present day celebrated as Valentines Day over the centuries since it is the first time Valentines Day is found "packaged" in such a manner, and probably in error (evidence the mating of birds referred to in the poem doesn't occur until spring). The saints day for a bishop of Genoa named Valentine is celebrated on May 2nd and this may be the saint's day Chaucer was referring to in the poem. It makes so much more sense to associate love with the flowers and rebirth of May Day, doesn't it?
The narrator seems confused by love and hits the books to try to understand the situation and ultimately fails. The poem is filled with historical allegory which is invisible to the modern reader who is unfamiliar with the court politics of the day, the major characters, and their motivations or relations to the King and his bride.
The following excerpt from eChaucer, a modern English translation online hosted by the University of Maine
And when this work was all brought to an end, Nature gave every bird his mate by just accord, and they went their way. Ah, Lord! The bliss and joy that they made! For each of them took the other in his wings, and wound their necks about each other, ever thanking the noble goddess of nature. But first were chosen birds to sing, as was always their custom year by year to sing a roundel at their departure, to honor Nature and give her pleasure. The tune, I believe, was made in France. The words were such as you may here find in these verses, as I remember them. >Welcome, summer, with sunshine soft, >The winter's tempest you will break, >And drive away the long nights black! >Saint Valentine, throned aloft, >Thus little birds sing for your sake: >Welcome, summer, with sunshine soft, >The winter's tempest you will shake! >Good cause have they to glad them oft, >His own true-love each bird will take; >Blithe may they sing when they awake, >Welcome, summer, with sunshine soft, >The winter's tempest you will break, >And drive away the long nights black!"