octave (n.)

c. 1300, utaves (plural, via Anglo-French from popular Old French form oitieve, otaves), reformed in early 15c., from Medieval Latin octava, from Latin octava dies "eighth day," fem. of octavus "eighth," from octo (see eight).

Originally "period of eight days after a festival," also "eighth day after a festival" (counting both days, by inclusive reckoning; thus if the festival was on a Sunday, the octaves would be the following Sunday).

Verse sense of "stanza of eight lines" is from 1580s; musical sense of "note eight diatonic degrees above (or below) a given note" is by 1650s, from Latin octava (pars) "eighth part." Formerly English eighth was used in this sense (mid-15c.)