Why is the ninth month called September?
early 13c., from an unrecorded Old English word or else from Old Norse gapa "to open the mouth, gape," common West Germanic (cf. Middle Dutch, Dutch gapen, German gaffen "to gape, stare," Swedish gapa, Danish gabe), from PIE *ghai- (see gap). Related: Gaped; gaping. As a noun, from 1530s.