No one can identify the first Latin lover to sing tender tunes to his sweetheart at dusk, but the custom was firmly established in early modern times.
Crooning on a rainy night was not desirable, so gallants hoped that the eventide would be clear and calm - which in Italian was serenata. No surprise, then, that love songs were common when serenata prevailed. In time, rain or shine, the melodies came to bear the name of serene summer evenings - serenades.
'X' marks the kiss
Our practice of putting an X at the end of a love letter to symbolize a kiss probably came from medieval documents. In early times, illiterates signed contracts and agreements with St. Andrew's cross and kissed the X to guarantee faithful performance of their obligation. 'Sealed with a kiss' made the documents binding.
Over time, X (an unknown quantity in mathematics) became known as 'the kiss,' and to some other theorists X is but the stylized picture of two mouths touching.
SOURCES: 'The Encyclopedia of Word and Phrase Origins,' by Robert Hendrickson; 'The Barnhart Dictionary of Etymology,' by Robert K. Barnhart; 'Why You Say It,' by Webb Garrison.