"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).

The "eighth day" or octava dies was associated with the weekly Christian celebration of the resurrection of Christ every "eighth day", which became a name for Sunday.

As circumcision is performed in Judaism on the “eighth day” after birth, the number 8 became associated in Christianity with Baptism, and baptismal fonts have from an early date often been octagonal.

The practice of octaves was first introduced under Constantine I, when the dedication festivities of the basilicas at Jerusalem and Tyre, Lebanon were observed for eight days. After these one-off occasions, annual liturgical feasts began to be dignified with an octave. The first such feasts accorded such were Easter, Pentecost, and in the East, Epiphany. This occurred in the 4th century and served as a time for the newly converted to take a joyful retreat.

The development of octaves occurred slowly. From the 4th century to the 7th century, Christians observed octaves with a celebration on the eighth day, with little development of the liturgies within the intervening days. Christmas was the next feast to receive an octave, and by the 8th century, Rome had developed liturgical octaves not only for Easter, Pentecost, and Christmas, but also for the Epiphany as in the East, and the feast of the dedication of a church.

From the 7th century, saints’ feasts also began to have octaves (as an eighth-day feast, not eight days of feasting), among the oldest being those of Saints Peter and Paul, Saint Lawrence and Saint Agnes. From the 12th century, the custom arose of liturgical observance of the days between the first and the eighth day, as well as the eighth day. During the Middle Ages, octaves for various other feasts and saints were celebrated, depending upon the diocese or religious order.

While Pope Pius V reduced the amount of octaves in 1568, these were still numerous. Not only on the eighth day from the feast but, with the exception of the octaves of Easter, Pentecost, and, to a lesser extent, Christmas, on all the intervening days the liturgy was the same as on the feast day itself, with the exact same prayers and Scripture readings.