Why is the ninth month called September?
Old English burg, burh "a dwelling or dwellings within a fortified enclosure," from Proto-Germanic *burgs "hill fort, fortress" (cf. Old Frisian burg "castle," Old Norse borg "wall, castle," Old High German burg, buruc "fortified place, citadel," German Burg "castle," Gothic baurgs "city"), from PIE *bhrgh "high," with derivatives referring to hills, hill forts, fortified elevations (cf. Old English beorg "hill," Welsh bera "stack, pyramid," Sanskrit bhrant-, Avestan brzant- "high," Greek Pergamos, name of the citadel of Troy).
In German and Old Norse, chiefly as "fortress, castle;" in Gothic, "town, civic community." Meaning shifted in Middle English from "fortress," to "fortified town," to simply "town" (especially one possessing municipal organization or sending representatives to Parliament). In U.S. (originally Pennsylvania, 1718) often an incorporated town; in Alaska, however, it is the equivalent of a county. The Scottish form is burgh. The Old English dative singular byrig survives in many place names as -bury.
in Great Britain, incorporated town with special privileges or a district entitled to elect a member of Parliament.