As a symbol of my own ignorance I have risen to #1 in the Google search engine when searching "St Swiggins Day", and actually I do get a lot of hits for it. Since the primary entry for this search actually didn't mention when St Swiggin Day was I thought I'd look it up to help my readers. This turned out to be more trouble than I first foresaw, I searched everywhere and yet I couldn't find him or it, I was on the verge of dragging out the Shakespeare when I thought to myself, "well, I can't remember how it was actually spelled so there's a high probability I've messed this up and others are as confused as I"... This turned to be the correct assertion as when I searched through the record of Saint's he was there all the time staring me in the eye.
St. Swiggins Day (actually this spelling appears to have come from a Simpson's episode) is actually Saint Swithun's Day. It occurs on the 15th of July, and legend dictates that if it rains on St Swithun's Day it will rain for the next 40 days, and if it is sunny, it will be sunny for the next 40 days.
Anyways, there it is, St Swithun's Day (AKA St Swiggin's Day) is July 15th, there's another reason to have a drink in the middle of summer, raise one to St Swiggin everyone.