Latin names of notes were invented by Guido d'Arezzo in XI century, using a hymn where the first verse started with C, the second with D and the seventh with B
Ut queant laxis
Resonare fibris
Mira gestorum
Famuli tuorum
Solve polluti
Labii reatum
Sancte Ioannes
The starting syllables of these verses gave the name at their notes: ut, re, mi, fa, sol, la. Si (B) was introducted later, and it was taken with the initials of Sancte Ioannes (it was a hymn for St. John) In XVI century ut became do.