Octave (liturgical)
"Octave" has two senses in Christian liturgical usage. In the first sense, it is the eighth day after a feast, reckoning inclusively, and so always falls on the same day of the week as the feast itself. The word is derived from Latin octava (eighth), with dies (day) understood. The term is also applied to the whole period of these eight days, during which certain major feasts came to be observed.
Octaves are not to be confused with eight-day weeks: see Christian "eighth day".
Read more about Octave (liturgical): From Origin To Middle Ages, From Pius V To Pius XII, Reduction By Pius XII and Paul VI, Eastern Christian Usage, Non-liturgical Usage