Sumer is icumen in

The British Library has lent for exhibition until May to the Reading Museum the manuscript produced at the great Benedictine abbey which once dominated the town, and which contains the mid-thirteenth century musical round - or canon - Sumer is icumen in.

This appears to be the earliest English song with music to survive. It is also interesting in that the words are in English from a time when it is often claimed that French was the dominant language of culture and music.

RdgToday has a report in the loan at Centuries-old manuscript returning to Reading after half a millennium away

Wikipedia has a lengthy and informative article about the piece at Sumer_is_icumen_in. It also includes some modern parodies which make for a little light relief.