Today (15 July) is St Swithin's Day (sometimes spelled Swithun, which scholars believe is closer to the Saxon pronunciation). The folk belief says that if it rains today it will rain for the next forty days, and if it is fine today, then it will be fine for the next forty days. Here's the rhyme:
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 na mair
I'm sorry to say that it rained all morning, although the afternoon has been gloriously sunny - so that confuses matters as regards St Swithin's prophecy.
The BBC published an article about the legend and I was intrigued by one of the comments made by a reader, which claimed that the number forty appears so often in folk tales and in the Bible because it is meant to signify 'a lot' rather than a specified number. The commentator said it was a mistranslation of the ancient Greek word for 'many'. The OED says forty is from an Old English word, and is similar to the equivalent word in other Germanic languages eg Frisian and Old Norse. However, one of the definitions is "Used indefinitely to express a large number".