Why is the ninth month called September?
mid-14c., "to go along with, properly relate to," from be- intensive prefix, + longen "to go," from Old English langian "pertain to, to go along with," of unknown origin. Senses of "be the property of" and "be a member of" first recorded late 14c. Cognate with Middle Dutch belanghen, Dutch belangen, German belangen. Replaced earlier Old English gelang, with completive prefix ge-.