I first heard of St. Swithun's Day through an expression I encountered in West Virginia. When an old farmer I knew wanted to express the opinion that something would probably never happen, he would say it would happen on St. Swithun's Day. My guess at the time was that he assumed that nobody knew when St. Swithun's Day was and so, in his mind, was equivalent to never. I don't know if the expression was used by others in the area.

He was also often known to say that something impossible would happen on the "31st of Juvember when the fall beans is ripe."