Will it or won’t it rain tomorrow? It is an important question because if it does you had better book your flight to the sun for your summer holiday. If not, don’t. For tomorrow is St Swithun’s Day — and rain then, of course, means more rain for 40 days thereafter.

The Victorians, who built St Swithun’s Quad at Magdalen College, between 1879 and 1884, chose the name well. For the college was founded by William of Wayneflete, Bishop of Winchester, in 1458, and St Swithun (800-862) was himself Bishop of Winchester for the last ten years of his life.

He was apparently a man of extraordinary humility, credited with miracles galore. He left instructions that he be buried outside Winchester Cathedral — where his grave would be trodden upon by the feet of passers by, and also be rained upon.

His instructions were carried out with all due ceremony. However, on his saint’s day in 971 his remains were moved inside the cathedral as a mark of respect — and then, of course, it rained for 40 days! Hence the rhyming proverb: “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 no mair.”

Well into the 20th century, there was also a belief that rain on St Swithun’s Day “blesses and christens the apples”, which should not be picked or eaten before his feast; and that all apples growing on July 15 will ripen and come to maturity.

One of St Swithun’s most famous miracles was to save Emma (988-1052), the Queen of England who gave the Bishop’s Palace at Witney to the Bishop of Winchester — with whom she was accused of having an affair.

Legend has it that the saint, nearly 200 years after his own death, interceded on her behalf when she was forced to walk barefoot across red hot ploughshares.

She emerged from the ordeal with unblistered feet, having spent the night praying at St Swithun’s shrine.

There is also a connection between St Swithun’s Day and that of St Mary Magdalene, whose feast day falls a week later. Another proverb has it that “rain today shows that St Mary Magdalene is washing her handkerchief” and adds that heavy rain on that day will spell a disastrous harvest.

St Mary Magdalene was, of course, the sinful woman who washed Christ’s feet. She is the patron saint of penitent women — and, in particular, reformed prostitutes.

A text called The Country Justice, of 1668, avers that the “usual canonical penance for a whore” was that she should “stand for some hours at a church door bare-legged and barefoot, in a white sheet, with a candle in her hand”. However it adds: “But this is now left off in many places, for they truly say, it but affordeth naughty women of their bodies occasion to display their wares, and entice men to filthiness.”

As for apples, there is yet another tradition that unless it rains on St Peter’s day (June 29) they will not ripen — but traditions are evidently muddled up together here.

As for the weather, some now say there is scientific evidence for St Swithun’s proverb. Until the middle of July, they say, there is a heavenly battle as to whether the jet stream will settle across the north or the south of Britain. If the former, high pressure will prevail; if not — it won’t.