(St.). If it rains on St. Swithin's day (15th July),there will be rain for forty days. (See Gervais.)
St. Swithin's day, gif ye do rain, for forty days it will remain; St. Swithin's day, an ye be fair, for forty days `twill rain nae mair.
The French have two similar proverbs - “S'il pleut le jour de St. Médan ” (8th June), “il pleut quarante jours plus tard; ” and “S'il pleut le jour de St. Gervais ” (19th June), “il pleut quarante jours après. ”
The legend is that St. Swithin, Bishop of Winchester, who died 862, desired to be buried in the church—yard of the minster, that the “sweet rain of heaven might fall upon his grave.” At canonisation the monks thought to honour the saint by removing his body into the choir, and fixed July 15th for the ceremony; but it rained day after day for forty days, so that the monks saw the saints were averse to their project, and wisely abandoned it. The St. Swithin of Scotland is St. Martin of Bouillons. The rainy saint in Flanders is St. Godeliéve; in Germany, the Seven Sleepers.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894