The words and lyrics of the nursery rhyme reflect the 'old wive's tale' that if it rains on St. Swithin's day then it will continue to rain for a further forty days.
St. Swithin's Day falls on 15th July. St. Swithin, or Swithun was born circa 800 and died AD862. He was a Saxon Bishop of Winchester and was originally buried, at his request, in a humble outside grave at Winchester. Nine years later the monks at Winchester moved his remains to a magnificent shrine inside Winchester cathedral on 15 July 971. Legend says that during the ceremony it began to rain and continued to do so for forty days. The Shrine of St. Swithun, together with the tomb of Alfred the Great, in Winchester Cathedral made the Cathedral a principal place of pilgrimage in England. The shrine was destroyed in 1538 by King Henry VIII' s men during the Dissolution of the Monasteries.
There are several rhymes of this nature, some of which also have significant historical relevance such as: