The solfege syllables (ut, re, mi, fa, sol, la, si) come from St. John's Hymn, in which the lyrics of the first verse go as:
Ut queant laxis
Resonare fibris
Mira gestorum
Famuli tuorum
Solve polluti
Labii reatum
Sancte Ioannes
The first six syllables - ut, re, mi, fa, sol, and la, are built on scale degrees 1, 2, 3, 4, 5, and 6 respectively. But the first note of the seventh line is scale degree 5.
In fact, the 7th note of the scale isn't found anywhere in the hymn.
