St. Swithin's Day is on July 15th.
St. Swithin was the Bishop of Winchester, England. He was buried on July 2, 862. He asked to be buried outdoors where his grave would be exposed to the rain. (Later his grave was moved indoors to a shrine).
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*.
*Na mair = no more
Simply put, this proverb means, if it rains on St. Swithin's Day it will rain for 40 days after. If it's nice weather on St. Swithin's Day, it will be fair for 40 days after.
This proverb goes back at least to 1599.
Thanks and Acknowledgements
This rhyme can be found in The Little Mother Goose (1912), illustrated by Jessie Willcox Smith.
Illustration from The Big Book of Nursery Rhymes (circa 1920) edited by Walter Jerrold (1865 – 1929) and illustrated by Charles Robinson.