The weather myth states that if it rains on July 15, wet weather will persist for 40 days and 40 nights.
So while people in England and Wales are in for a glorious dry summer those in Scotland and Northern Ireland might not be so lucky.
But Met Office forecaster Rachael West allayed fears as she said people who have enjoyed a dry Sunday will probably see some showers over the next week, and vice versa.
“Certainly some changes in forecast over the next week, never mind the next coming 40 days. But if people want to believe in St Swithin’s Day then that’s up to them,” she joked.
St Swithin was an Anglo-Saxon bishop of Winchester, who died in AD862.
When he was made a saint in 971, his body was dug up and moved to an indoor shrine in the city’s cathedral.
Some writers claimed this outraged the heavens, causing rain to pour on the church and continue uninterrupted for 40 days.
But the Met Office said there had not been a record of 40 dry or 40 wet days following St Swithin’s Day since records began in 1861.
Ms West said some people are probably pleased to see the rain, especially those in Northern Ireland where there is a hosepipe ban.
Bookmaker Ladbrokes believe there is a 100/1 outside chance that rain will fall consecutively for 40 days and 40 nights, and Jessica Bridge from the bookie said: “We’re hoping and praying that the odds are on the money.
“While the heatwave may not last all summer, it looks like moderate temperatures and sunshine will instead of rainfall.”
Temperatures could potentially soar to 31C or 32C on Sunday and Monday, probably in the London area.
As the week goes on, it is expected to get a little cooler, and not as hot as it has been.