Today, the feast of the birth of St John the Baptist, is, in a way, even more a musicians' feast than the feast of St Cecilia. Why, is a bit interesting.
The hymn at both Vespers (in both ordinary and extraordinary forms) goes as follows:
Ut queant laxis
Free from guilt your servants' unclean lips, O holy John, that they may be able to sing with clear voices the the wonders of your life.
Here's the music:
Now, I suppose, if you're not in any way musical, this will pass you by, but if you look at all the notes accompanying the first syllables at each of the half bars, thus, at UT, REsonare, MIra, FAmuli &c, you'll see that each note is one degree higher than the last, with the exception of the last.
Now put the syllables together, and you get:
Ut, re, mi, fa, so, la.
This comes down to us as the 'scale' or 'steps' of music, sometimes known as the scala aretina, or the hexachord. Several settings of the Mass use this as a cantus firmus (a harmonic base, and/or a recurring theme in the music), such as the English Tudor composer Avery Burton's Missa Ut, re, me, fa, sol, la, or the better known Francisco Valls' Missa Scala Aretina.
And when people thought that 'ut' was a bit ugly, they substituted 'do' instead, adding also 'ti' to make a full scale. Which leads us back to………