"Sumer Is Icumen In" is a traditional English medieval round, and possibly the oldest such example of counterpoint in existence. The title might be translated as "Summer has come in" or "Summer has arrived".
The round is sometimes known as the Reading rota because the manuscript comes from Reading Abbey though it may not have been written there. It is the oldest piece of six-part polyphonic music (Albright, 1994). Its composer is anonymous, possibly W. de Wycombe, and it is estimated to date from around 1260. The manuscript is now at the British Library. The language is Middle English, more exactly Wessex dialect.
(From the YouTube video site.)
Summer has arrived,
Loudly sing, Cuckoo!
The seed grows and the meadow
And the wood springs anew.....etc.
A few, more modern, celebrations of the season: