The rhyme:

"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 reason:

St Swithin's Day falls on July 15 every year when the proverb states that if it rains on that day it will rain for another 40 days and 40 nights. On his deathbed, he asked to be buried in a humble grave outside so that the "sweet rain from heaven" could fall on his grave.

But on July 15, 971, Monks decided to move his grave into a shrine. Legend has it that torrential rains poured down on the day he was moved and for the following 40 days and 40 nights. Apparently it was St Swithin showing his displeasure by weeping in despair.