Perhaps a reason for Valentine's Day to fall on February 14th is the long held tradition that birds select their mates on that date. Literature shows that Europeans believed that on February 14th, the birds began to choose their mates.
Consider Chaucer's "Parlement of Foules,"
in which he wrote:
"For this was Seynt Valentine's Day
when every foul cometh ther
to choose his mate."
From the writing of John Donne:
"Hail Bishop Valentine! whose day this is;
All the air is thy diocese,
And all the chirping choristers
And other birds are thy parishioners;
Thou marryest ever year
The lyric lark and the grave whispering dove;
The sparrow that neglects his life for love,
The household bird with the red stomarcher;
Thou mak'st the blackbird speed as soon,
As doth the goldfinch or the halcyon ...
This day more cheerfully than ever shine,
This day which might inflame thyself,
The tradition of drawing names on St. Valentine's Eve continued in England and other European locals. The idea that birds chose their mates on St. Valentine's Day led to the tradition that humans would do the same.
When a young man drew a girl's name, he would wear it on his sleeve, and would be expected to attend her needs during the following year. The girl thus became his valentine and they exchanged love tokens.
The custom of gift-giving later evolved into the idea that only the male would give the gifts, usually without names, but signed, "With St. Valentine's Love."
In France, both sexes drew from the valentine box. The book, "Travels in England," (1698), gives this account:
"On St. Valentine's Eve, an equal number of Maids and Bachelors get together, each writes their true or some feigned name upon separate billets, which they roll up and draw by way of lots ... the maids taking the mens billets, and the men the maids, so that each of the young men lights upon a girl that he calls his Valentine, and each of the girls upon a young man which she calls hers.
This means each has two Valentines ... but the man sticks faster to the Valentine that is fallen to him than to the Valentine to whom he is fallen. Fortune having thus divided the company into so many couples, the valentines give balls and treats to their mistresses, wear their billets several days upon their bosoms or sleeves, and this little sport ofen ends in Love.
This ceremony is practiced differently in different countries, and according to the freedom or severity of Madame Valentine. This is another kind of Valentine, which is the first young man or woman chance throws in your way in the street, or elsewhere."
Shakespeare also wrote of St. Valentine's Day in a Midsummer Night's Dream. A character in the play discovers two lovers in the woods and asks:
"St. Valentine is past; Begin these woodbirds but to couple now?"
The poet, Drayton, wrote the verses, "To His Valentine," in which the idea of the birds mating on St. Valentine's Day was mentioned:
"Each little bird this tide
Doth choose her beloved peer,
Which constantly abide
In wedlock all the year."