St Swithun was a bishop of Winchester, UK, in AD862. The legend has it that on his deathbed he made them promise to bury him in the churchyard, amongst Gods' creation, under Gods' sky or else he would visit them with a curse of foul weather. The local bigwigs decided a few years later that the churchyard was no fit resting place for a bishop and moved him into the catherdral. On the day his bones were translated, there was a torrential downpour that lasted all day. Allegedly, it rained every day until the bigwigs gave up and, for the sake of the ruined harvest, relocated him to his former resting place whereupon the rain stopped and all was shiny and bright again. Hence the reputation of St Swithun for miracles which lead to his canonisation.

40 days is an arbitrary figure, in the Bible, 40 is used frequently but it is probably a corruption of the word for many.

HOWEVER:.... in Britain there is quite often a period of settled weather, starting in the middle of July and lasting for about 40 days... just up until the August Bank Holiday in fact. The weather could be settled as sunny, windy or rainy, but it would be fairly settled.

The same sort of miracle is attributed to several saints, particularly St Winifred (yes, I know it's in an Ellis Peters' crime novel but it's based on legend), whose procession remained dry throughout the town whilst the rest of the town were soaked by a long shower.