The myth goes that if it rains on July 15th, wet weather will continue for 40 days and 40 nights.
St Swithin was an Anglo-Saxon bishop of Winchester.
He died in AD862 and was made a saint in 971.
His body was dug up and moved to an indoor shrine in the city's cathedral, and some writers claimed this outraged the heavens, causing rain to pour on the church and continue uninterrupted for 40 days.
The weather forecast for Friday predicts heavy rain for the north of the country and south is expected to be cloudy but largely dry, according to the Met Office.
Meteorologist Emma Sharples said: "It's different north to south tomorrow - most places have been fine today but we have got a weather system that is going to move into the north of the UK."
Ms Sharples said that wet weather will hit Northern Ireland, western Scotland and northern England early in the morning, while further south will be cloudy but with patchy rain.
She said: "The Midlands southwards should really stay dry throughout tomorrow but it will be becoming an increasingly cloudy picture there. The best of any brightness is going to be in the south east.
"If you believe the saying, 40 days and 40 nights of rain in the north to come, and 40 days of staying dry in the south but I think we know that's probably not going to come true."
The Met Office said there had not been 40 dry or 40 wet days following St Swithin's Day since records began in 1861.