'St Swithin's Day, if thou dost rain, For forty days it will remain; St Swithin's Day, if thou be fair, For forty days 'twill rain na mair.' The legend connecting St Swithin's Day with the weather dates back to 15 July 971, when the bones of St Swithun, bishop of Winchester (the Dictionary of National Biography firmly castigates 'Swithin' as a misspelling) were scheduled to be moved from an unmarked grave outside his church to a consecrated site within the walls of the building. On that day, however, it is said to have poured with rain, and continued to do so for the next 40 days, which was taken as a sign that his bones preferred to remain where they were.
That account, however, seems to be a piece of 16th-century romanticism at variance with contemporaneous accounts of the event. Back in the 10th century, it was generally thought that Swithun had been overjoyed at having his bones reburied, and he was credited with hundreds of miracles supposedly performed in gratitude.
The long-range weather forecast associated with Swithun also fails to stand up to scrutiny. An analysis was published in 1894 calculating the number of rainy days in the 40 days following 15 July. After a wet St Swithun's Day, there were an average of 18.5 rainy days, while for a dry 15 July, the rainy score was slightly higher at 19.25.
Whatever the statistics show, St Swithun did give Britain its own meteorological saint's day to match those of St Medard (8 June) in France, St Godelieve (6 July) in Belgium, and the Day of the Seven Sleepers (27 June) in Germany, all of which carry a similar tradition of 40-day, long-range weather forecasts. And none of those works either. In fact, records suggest that it has not rained for forty days since the Great Flood.