St. Swithin’s Day if thou dost rain,
For forty days it will remain.
St. Swithin’s day if thou be fair,
For forty days ’twill rain nae mair.
Saint Swithun (or Swithin) was an Anglo-Saxon bishop of Winchester and subsequently patron saint of Winchester Cathedral. His historical importance as bishop is overshadowed by his reputation for posthumous miracle-working. According to tradition, if it rains on Saint Swithin’s bridge (Winchester) on his feast day (15 July) it will continue for forty days. (Wiki)