Why is the ninth month called September?
1650s, from un- (1) "not" + past participle of foresee. Cf. Middle Dutch onvoresien, Dutch onvoorzien, Middle High German unvorsen.
Old English foreseon "have a premonition," from fore- "before" + seon "to see, see ahead" (see see (v.)). Related: Foresaw; foreseeing; foreseen.