In our lands, the first beautiful days of early spring sometimes appear in mid-February devoted to the famous martyr, the old calendars, able to mix seasons and religion, astronomy and liturgy, represented by a sun the return of Saint- Valentin on the cycle.

After a hard winter, our fathers prepared themselves for the next summer months by observing scrupulously the popular saying written on the 14^th of February in the almanacs of the good old days :

"Bleeding the day Saint-Valentin
Get the blood clear night and day
And the bleeding of the day before
Keep fevers the whole year"

Some people explained that Saint Valentin was claimed as Patron of lovers as far back as the Middle Ages. At that time, they asserted that on February 14^th, one opinion gained currency, especially in France and in Great Britain, the birds began to mate. "In mid-February, a good blackbird must build its nest" according to the proverb and, this day, every Valentin had to choose his Valentine...

Some pretended that we found ourselves here in presence of a common morphological phenomenon : by assimilation of the V to G (as Vedalus of February the 6^th gave Gaston, Vitus of June the 15^th became Guy…).
Valentin and Valentine must naturally become Galentin and Galentine, hence the small courteous festival of February 14^th .

Is true reason a different thing ?
The folklore is very close to life to be satisfied with a dry answer.