St Swithin (or St Swithun) was the Bishop of Winchester about 1200 years ago during the reign of King Egbert. He was especially keen on having new churches built in places that didn’t already have a church, and he repaired those that had been damaged or destroyed.
He died in 862 AD and on his deathbed he asked that he could be buried in the grounds of Winchester Cathedral “where the rain would fall on him and the feet of ordinary men could pass over him.” However, over a hundred years later he was pronounced the patron saint of Winchester, and Bishop Aethelwold decided that he should have a more fitting shrine inside the basilica.
On the 15th July 971 a ceremony was held to dig up his remains and move them into the new shrine (Henry VIII had the shrine destroyed in around 1538). As the ceremony proceeded there was an enormous downpour and it then continued to rain for 40 days, which the monks took as a sign of the saint’s displeasure. This it is believed was the start of the aphorism.
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 na mair.
Sadly this is not a particularly accurate meteorological method. Since records began there have never been 40 consecutive days of rain, although there have been 4 years in the last 150 years where there has almost been 40 dry days following a dry day on 15th July.
More generally though, the meteorological situation around the end of June to early July, indicated by the position of the jet stream, does determine the general weather patterns (hot, cold, dry, wet) for the rest of the summer. So the more southerly the location of the fronts the more likely we are to have unsettled, wet and cool weather. If the fronts are further north high pressure will build over western Europe, resulting in dry, warm weather in the UK.
The UK is the only country which calls July 15th St.Swithin’s day, but similar sayings exist in several European countries (France, Germany, Belgium, Holland), but the only other country which has 15th July for the start of 40 days is Holland – St Henricus day.