St Swithun's Day - 15 July
St Swithun was a 9th century Anglo-Saxon bishop of Winchester. You can find a short biography in the good old Catholic Encyclopædia here. Wikipedia has something here and a schools page has something of his place in British custom and tradition here.
The good bishop ought to be better known in southern California as he is the patron of those places enduring drought. The rhyme gives it as fact that whatever the weather on his day it will continue for 40 days.
St Swithun's Day if thou doest rain
For forty days it will remain
St Swithun's day if thou be fair
For forty days twill rain nae mair.
So drought alleviation petitions probably ought to be made prior to the feast day.
In fact, he's one of those saints associated with all sorts of weather patterns. Saith the Wikipedia page;
In France, Saint Medard (8 June), Urban of Langres, and Saint Gervase and Saint Protais (19 June) are credited with an influence on the weather almost identical with that attributed to St Swithun in England. In Flanders, there is St Godelieve (6 July) and in Germany the Seven Sleepers' Day (27 June).
All medieval stuff and nonsense. Or maybe not. The same page continues:
Around the middle of July, the jet stream settles into a pattern which, in the majority of years, holds reasonably steady until the end of August. When the jet stream lies north of the British Isles then continental high pressure is able to move in; when it lies across or south of the British Isles, Arctic air and Atlantic weather systems predominate.There is apparently and addendum of sorts to the traditional rhyme which advises:
If on St Swithun's Day it really pours,
You're better off to stay indoors.
Belabours the obvious somewhat, but who am I to disagree with a traditional rhyme?