Why is the ninth month called September?
early 15c., from Old French (danse) Macabré "(dance) of Death" (1376), probably a translation of Medieval Latin (Chorea) Machabæorum, literally "dance of the Maccabees" (leaders of the Jewish revolt against Syro-Hellenes; see Maccabees). The association with the dance of death seems to be via vivid descriptions of the martyrdom of the Maccabees in the Apocryphal books. The abstracted sense of "gruesome" is first attested 1842 in French, 1889 in English.
The typical form which the allegory takes is that of a series of pictures, sculptured or painted, in which Death appears, either as a dancing skeleton or as a shrunken corpse wrapped in grave-clothes to persons representing every age and condition of life, and leads them all in a dance to the grave. ["Encyclopaedia Britannica," 11th ed., 1911]