Gotcha! You were expecting some deep, historical insight, weren't you?
Today is St Swithun's Day. Or St Swithin's Day, if you prefer. I don't suppose he minds over much which way his name is spelt.
St Swithun's day if thou dost rain
For forty days it will remain.
St Swithun's day if thou be fair
For forty days 'twill rain nae mare.
A Buckinghamshire variation has
If on St Swithun's day it really pours
You're better off to stay indoors.
So far so good.