St. Swithin was an Anglo-Saxon Bishop. He served as Bishop of Winchester from 30 October 852 until his death on 02 July 862. St. Swithin was known more for his posthumous miracle-working than for what he accomplished while alive.
The significance of St. Swithin's Day is the belief that if it rains on that day, it will continue to rain for forty days.
It has been claimed that the prognostication is derived from an actual event. St. Swithin insisted on being buried outside in the open churchyard of Winchester Cathedral. But the monks felt that the Saint should have been buried in the chancel of the minster, as were all the other bishops. So on the 15th of July in the year 865, the year Swithin was canonized by the Pope, the monks of Winchester undertook to dig up the remains of the Saint. A violent storm arose and the resulting rain continued for forty days, forcing the monks to give up their intentions. Instead of moving the Saint's remains into the Cathedral, a new chapel was built over the remains.
Poor Robin's Almanack for the year 1735 presented its readers with the following verse:
If it rains on St. Swithin's Day;
I've heard some antient farmers say
It will continue forty days,
According to the country phrase.
'Tis a sad time, the lawyers now,
And doctors nothing have to do,
Likewise the oyster women too.