In the eighth century, a Monk of Monte Cassino, Paulis Diaconus (Paul the Deacon) composed the hymn "Ut queant laxis" in honour of St John the Baptist (Feast Day celebrated last week on 24 June).
It is now a rather famous piece of musical history as it formed the basis for the teaching of musical scales.
In the thirteenth century the Benedictine monk Guy of Arezzo noticed that the notes sung on the first syllables formed the sequence of the first six degrees of the scale. He named each degree by the corresponding syllable: "Ut, re, me, fa, sol, la si":
Ut queant laxīs resonāre fibrīs (Do-re)
Mīra gestōrum famulī tuōrum (mi-fa)
Solve pollūtī labiī reātum (Sol-la)
Sāncte Iōhannēs. (Si)
Whilst, then, Rodgers and Hammerstein created and popularised "Do-Re-Mi" in the 1965 film Sound of Music, the history is far richer!