“St. Swithin'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.”
St. Swithin (sometimes as "Swithun" by clever dicks) (c.800 - c.863) was bishop of Winchester Cathedral. Like all Christian saints, one or two crap miracles have been attributed to him to fill out the calendar with feast days. St. Swithin did like to build churches and should receive credit for that, but it's not like he rolled up his sleeves and set to work with masonry tools.
His big miracle was restoring an old lady's smashed eggs, assumed to be from her chickens and not her own, biologically. The doughty bishop just happened to have his street magician kit with him at the time, and with a wave of his wand and hey presto!, the broken eggs were whole again. Some reports instead had him running a 3 Card Monte table set up next to a church he was having built at the time, implying that sleight of hand rather than supernatural forces came into play. Happily, blasphemers telling that tale were burnt at the stake and the first story has become canon. Pun intended.
His feast day is July 15. Note the weather that day, for as the poem above states, the weather on that day will predict the weather for the next forty days, the good biblical number. This has correctly reported the weather at least twice in the nearly 1200 years since his death. We need not question why this has happened, only that it is a miracle guaranteed not to spoil any 2 holidays in 1200 taken during this period.