While this day and the fairly recent custom of sending cards, flowers, chocolates and other gifts is now closely associated with love, there have been traditions in continental Europe and in the United Kingdom that are still connecting the saint with the arrival of spring.
In Slovenia, Saint Valentine was one of the saints of spring, the saint of good health and the patron of beekeepers and pilgrims. Plants and flowers start to grow on the 14th February. It has been celebrated as the day when the first work in the vineyards and in the fields starts.
A tradition in Norfolk could be a link between the initial origin and today's custom of giving love offerings. A character called 'Jack Valentine' knocks on the rear door of houses leaving sweets and presents for children. Although he was leaving treats, many children were scared of this mystical person.
So only one thing is proven: Valentine's Day has only recently been celebrated as the day of love. Up to then, it was up to other saints to be the patrons of lovers, like for example St. Anthony and St. Gregory.
So Valentine's Day is a great example for the ability of traditions to change!