Octave in liturgical usage has two senses. In the first sense, it is the eighth day following a major feast (counting the feast itself as the first day), particularly in the Catholic, Eastern Orthodox and Anglican liturgical calendars. In the late Middle Ages the second sense of octave developed as an eight-day festival with a specific liturgy.
Western usage
The term is used in both senses today. In the former sense the Octave of Christmas among the Western churches always falls on January 1st and in the Catholic Church is the Solemnity of Mary, Mother of God, traditionally also celebrated as the Circumcision of Jesus, a holy day of obligation in many countries. Until 1955, the Roman Catholic calendar had many octaves, including the Immaculate Conception, Saint Joseph, Ss. Peter and Paul, Corpus Christi, the Sacred Heart, the Assumption, and All Saints, as well as, locally, the patron saint of a particular nation, diocese, or church, but all except Christmas, Easter, and Pentecost were suppressed. Traditional Roman Catholics keep all the feasts as they were prior to the changes stemming from the Second Vatican Council. They are found under the heading of Traditional Catholic Calendar. In the second sense, that is an eight-day celebration, the term is most commonly used today to refer to the Octave of Easter, the week (and particularly the Sunday) following Easter Sunday, which is the most vividly celebrated octave in the Catholic liturgical year. Each of the days in the Octave of Easter is celebrated with the rites of Easter Sunday, a period sometimes referred to as Easter Week, or the Week of Sundays. The Sunday following Easter Sunday is also called the Octave of Easter, meaning "the eighth day."
Eastern Christian usage
Among the Eastern Orthodox and Eastern Catholic Churches, what in the West would be called an Octave is referred to as an Afterfeast. The celebration of the Great Feasts of the church year are extended for a number days, depending upon the particular Feast. Each day of an Afterfeast will have particular hymns assigned to it, continuing the theme of the Feast being celebrated. Most of these Great Feasts also have a day or more of preparation called a Forefeast (those Feasts that are on the moveable Paschal Cycle do not have Forefeasts). Forefeasts and Afterfeasts will affect the structure of the services during the Canonical Hours. The last day of an Afterfeast is called the Apodosis (lit. "giving-back") of the Feast. On the Apodosis, most of the hymns that were chanted on the first day of the Feast are repeated. On the Apodosis of Feasts of the Theotokos, the Epistle and Gospel of the Feast are repeated again at the Divine Liturgy.
