If you don't already know, St Swithin's Day is 15 July, a day on which people watch the weather for tradition says that whatever the weather is like on St. Swithin's Day, it will continue so for the next forty days.
There is a weather-rhyme is well known throughout the British Isles since Elizabethan times.
'St. Swithin's day if thou dost raindost = does
For forty days it will remain
St. Swithin's day if thou be fair
For forty days 'twill rain nae mair.'
thou = you
nae mair = no more.