'Monkey's birthday' is a term used to describe a sun shower.... and curiously it is a Zulu phrase that seems to have become widespread in English. There are lots of variants around... a fox's wedding... a hyena is giving birth ... the rats are getting married..... in Flemish they say: 'They're having a party in hell.... in GeorgiA: 'The sun is washing it's face'.. They appear to be lighthearted phrases to describe the unusual.