Some of this depends on your country's tradition. In France and Italy, ABCDEFG = La Si Ut Re Mi Fa Sol. In Italy Ut is generally do. Germany also uses ABCDEFG.
(Also confusin'- B minor = German h-moll or b-moll; French si mineur, etc.)
Ut was the first note in Guido's hexachords. Showed up in print in 1325 (OED). Do first appeared in 1754, in Dict. Arts and Sciences, "A note of the Italian scale, corresponding to ut of the common gamut."
--"Used [in place of ut] to denote the first note (key note) of the scale (movable Do); or in some cases the note C, the key-note of the natural scale (fixed Do)."
"Said to have been the invention of G. B. Doni, who died 1669" (Groves 1881, in OED).

Si, in Chamber's Cyc., 1728. "In music, a seventh note, added in these 60 years, by one le Maire, to the Six ancient notes." (OED)
From Sancti Ioannis- 1850, Helmore in Plainsong. (OED)
1875, in Staines and Barrett, Dict. of Musical Terms "Here was a sa for the seventh note of the scale...but it was not employed. In later use, in order to mark another semitone by the final I (as in mi), sa was turned into si." (OED). Take your pick.