Why is the ninth month called September?
c.1300, verbal noun from follow (v.). Meaning "a body of disciples or retainers" is from mid-15c.
Old English folgian, fylgan "follow, accompany; follow after, pursue," also "obey, apply oneself to a practice or calling," from West Germanic *fulg- (cf. Old Saxon folgon, Old Frisian folgia, Middle Dutch volghen, Dutch volgen, Old High German folgen, German folgen, Old Norse fylgja "to follow").
Probably originally a compound, *full-gan with a sense of "full-going;" the sense then shifting to "serve, go with as an attendant" (cf. fulfill). Related: Followed; following. To follow one's nose "go straight on" first attested 1590s. "The full phrase is, 'Follow your nose, and you are sure to go straight.' " [Farmer].