The Collaborative International Dictionary of English v.0.48:

Valentine \Val"en*tine\, n.
   1. A sweetheart chosen on St. Valentine's Day.
      [1913 Webster]

   2. A letter containing professions of love, or a missive of a
      sentimental, comic, or burlesque character, sent on St.
      Valentine's Day.
      [1913 Webster]

   St. Valentine's Day, a day sacred to St. Valentine; the
      14th of February. It was a very old notion, alluded to by
      Shakespeare, that on this day birds begin to mate. Hence,
      perhaps, arose the custom of sending love tokens at that
      time.
      [1913 Webster]