According to tradition, whatever the weather is like on St Swithin's Day – whether rainy or sunny – it will continue for the next 40 days and 40 nights.
St Swithin (or St Swithun) was an Anglo-Saxon bishop at WinchesterCathedral who died in 862. At that time Winchester was the capital of the kingdom of Wessex.
He is believed to have been a trusted counsellor of Egbert, King of the West Saxons, and educated his son Ethelwulf who appointed him bishop.
Very little is known about his life and there's hardly any mention of him in documents from the time that he lived. However he was famed for his charity and church building and was made patron saint of Winchester Cathedral about 100 years after his death.
He supposedly performed just one miracle during his lifetime - making an old lady's eggs whole again after workmen smashed them while building a church.
Swithin is derived from the Old English word for "strong" and St Swithin's symbols are raindrops and apples.
Where did the legend originate?
As he lay on his deathbed St Swithin asked to be buried outside the Old Minster in Winchester, in a lowly grave where his body would be trodden and rained on.
However more than a century later, on July 15 971, Winchestermonks moved his remains to an elaborate shrine inside the cathedral where pilgrims flocked, believing his bones to have miraculous healing properties.
But legend has it that St Swithin wasn't happy about his body being moved. On the day of the removal, ferocious and violent rain storms arrived lasting 40 days and nights which apparently represented his displeasure.
This story soon became folklore and now British people keep an eye on the weather on July 15. The superstition is expressed in the well-known rhyme.
St Swithin’s Day, if it does rain
Full forty days, it will remain
St Swithin’s Day, if it be fair
For forty days, t’will rain no more
Nothing remains of St Swithin's shrine which was destroyed during King Henry VIII's Reformation, but there's a memorial to him at Winchester Cathedral.
Other western European countries observe a similar day, dedicated to different saints.
In France, people keep watch for rain on July 19 (St Gervais' Day) and Germany's Seven Sleepers day (July 7) refers to the weather patterns over the next seven weeks.
Thanks to the Daily Telegraph