"Octave" has two senses in Christian liturgical usage. In the first sense, it is the eighth day after a feast, counted 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) implied and understood. In the second sense, the term is applied to the whole eight-day period, during which certain major feasts came to be observed.
Octaves, not being successive, are quite distinct from eight-day weeks and simply refer to the return of the same day of a seven-day week in the inclusive counting system used in Latin (just as the ninth day was a return to the same day of a nundinal cycle, the eight-day week of the pre-Christian Roman calendar).
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