Why is the ninth month called September?
mid-14c., from Old French gable "facade, front, gable," from Old Norse gafl "gable, gable-end" (in north of England, the word is probably directly from Norse), probably from Proto-Germanic *gablaz "top of a pitched roof" (cf. Middle Dutch ghevel, Dutch gevel, Old High German gibil, German Geibel, Gothic gibla "gable"), from PIE *ghebhel.
Cognates seem to be words meaning both "fork" (cf. Old English gafol, geafel, Old Saxon gafala, Dutch gaffel, Old High German gabala "pitchfork," German Gabel "fork;" Old Irish gabul "forked twig") and "head" (cf. Old High German gibilla, Old Saxon gibillia "skull").
Possibly the primitive meaning of the words may have been 'top', 'vertex'; this may have given rise to the sense of 'gable', and this latter to the sense of 'fork', a gable being originally formed by two pieces of timber crossed at the top supporting the end of the roof-tree." [OED]Related: Gabled; gables.