July 15th commemorates the Bishop of Winchester, advisor to King Egbert, who died in 862.
Legend has it that on his deathbed, he asked to be buried outside where rain might fall on his grave. Many years later, on July 15, 931 (or 971), when Swithin was to be canonized and moved inside the church, 40 days of rain either delayed the proceedings or blocked them altogether.
Thus the following rhyme has come to be associated with the day:
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.