Of course, it's not a day marking some event at St. Mirren Football Club, rather the day to commemorate Saint Mirren himself. Sadly, the Scottish parliament don't seem to be prepared to have a public holiday to mark this occasion.
You can read lot's of info about St. Mirren at http://www.timesandstar.co.uk/features/viewarticle.aspx?id=412492 although I'm not sure if the part about him being the patron saint of football is accurate.