According to wiki:
Saint Valentine (in Latin, Valentinus) is a widely recognized third century Roman saint commemorated on February 14 and associated since the High Middle Ages with a tradition of courtly love.
什么是courtly love呢? Wiki says:
Courtly love or domnei was a medieval European conception of nobly and chivalrously expressing love and admiration.[1] Generally, courtly love was secret and between members of the nobility.[2]
It was also generally not practiced between husband and wife所以情人节是小二小三的节日...