Why is the ninth month called September?
Old English monig, manig "many, many a, much," from Proto-Germanic *managaz (cf. Old Saxon manag, Swedish mången, Old Frisian manich, Dutch menig, Old High German manag, German manch, Gothic manags), from PIE *menegh- "copious" (cf. Old Church Slavonic munogu "much, many," Old Irish menicc, Welsh mynych "frequent," Old Irish magham "gift"). Pronunciation altered by influence of any (see manifold).
Old English menigu, from many (adj.). The many "the multitude" attested from 1520s. Cf. also Gothic managei "multitude, crowd," Old High German managi "large number, plurality," German Menge "multitude."