St Swithun’s Day (July 15): when will the rain end?

St Swithun’s day if thou dost rain
For forty days it will remain
St Swithun’s day if thou be fair
For forty days ‘twill rain nae mare

St Swithun, Benedictional of Æthelwold, London

"The St Swithun's Day legend is an old one –the earliest surviving written reference dates back to the 14th century – although its roots are much disputed."

https://www.rmets.org/weather-and-climate/weather/st-swithins-day