According to The History Channel's web page here the origin is murky. One story is a priest in third century Rome continued to marry couples when the emperor outlawed marriage. When Claudius discovered Valentine's acts, he ordered him killed.
One legend is that an imprisoned man named Valentine sent the first valentine from his cell. Supposedly, he wrote a girl he loved a letter before his death and signed it "From your Valentine".
According to one website, the first commercial valentine was sent in the 1800s. According to the Greeting Card Association, one billion valentines are sent each year and it's the second biggest card-sending holiday after Christmas.
These days we keep Valentine's very low key, especially now that we're retired and living on a fixed income. I personally think how you're treated every day throughout the year is more important than an expensive gift on Valentine's. The day hasn't really changed since I started writing romance.
So, how do you and your spouse/significant other celebrate the day? Has the day changed for you since you started writing romance?