Why is the ninth month called September?
1731, from Medieval Latin rotarius "pertaining to wheels," from Latin rota "a wheel, a potter's wheel; wheel for torture," from PIE root *roto- "to run, to turn, to roll" (cf. Sanskrit rathah "car, chariot;" Avestan ratho; Lithuanian ratas "wheel," ritu "I roll;" Old High German rad, German Rad, Dutch rad, Old Frisian reth, Old Saxon rath, Old Irish roth, Welsh rhod "carriage wheel"). The international service club (founded by Paul P. Harris in Chicago in 1905) so called from the practice of clubs entertaining in rotation. Hence Rotarian (1911).