Here's a summary of the Wiki page that I've copied from an old blog

Ut queant laxis    

Resonare fibris

Mira gestorum    

Famuli tuorum,

Solve polluti    

Labii reatum,

Sancte Iohannes.

"Ut" was changed in the 1600s in Italy to Do, at the suggestion of the musicologist Giovanni Battista Doni (based on the first syllable of his surname). In Anglophone countries, "Si" was changed to "Ti" by Sarah Glover in the nineteenth century so that every syllable might begin with a different letter.

The original Ut was the note A. When G below it was added, it was called GammaUt, which gives us the word "gamut".

Nowadays Do always means "C" (e.g. "Fa dièse" is French for F#), except in countries where it just means the same as "tonic" (a friend went on holiday to Turkey once and met some musicians and asked what notes they tuned their instruments to. All he could get from them was "Do")

https://en.wikipedia.org/wiki/.....lf%C3%A8ge