Je suis desja d'amour tanné
Ma tres doulce Valentinée
Which translates to "I am already tired of love my very sweet Valentine."
(As an aside, the Battle of Agincourt was fought on Oct 25, 1415 when an outnumbered English army under the leadership of Henry V defeated the French who were trying to block the English retreat to Calais. It was one of the most important victories of the English during the 100 Years War. This was the battle about which Shakespeare wrote the St. Crispin's Day Speech in Henry V. As an an additional aside, Charles, Duke of Orleans, was a prolific medieval poet, producing more than 500 poems in English and French while imprisoned for 24 years following Agincourt.)
The earliest known English language Valentine greeting was from Margery Brews to her fiance John Paston. In the letter Margery refers to John as her "right well-beloved Valentine."
But perhaps the most well known Valentine's greeting began as a nursery rhyme in 1784 -
The rose is red, the violet's blue,
The honey's sweet, and so are you.