A saint's day was, simply one particular day set aside for a particular saint.
Saints' days were selected based on the date a person died. If two saints had died on the same day, the lesser saint's date was normally moved to the nearest open date on the calendar.
In the British Isles the saints of the continent as well as the numerous Celtic saints numbered in the hundreds, so that by the end of the Middle Ages there were very few days that were not dedicated to one or another saint. Days dedicated to saints called for a special reference to that particular saint and whatever was his most remembered associations - travel, commerce, virgins, etc.