In Dulci Jubilo
In Dulci Jubilo: this is one of my favourite carols (here performed by King's College, Cambridge). This type of carol is called 'macaronic' - the text is a mix of Latin and vernacular (another example is the Boar's Head). In Dulci Jubilo first appeared in manuscript from around 1400, but its origins are normally dated to Blessed Henry Suso's vision one night in 1328. According to an early life of the Dominican mystic:
Now this same angel came up to the Servant brightly, and said that God had sent him down to him, to bring him heavenly joys amid his sufferings; adding that he must cast off all his sorrows from his mind and bear them company, and that he must also dance with them in heavenly fashion. Then they drew the Servant by the hand into the dance, and the youth began a joyous song about the infant Jesus, which runs thus: In dulci jubilo etc.