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.
[5][6][7] 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.
[1] This occurred in the 4th century and served as a time for the newly converted to take a joyful retreat.
[1] During the Middle Ages, octaves for various other feasts and saints were celebrated, depending upon the diocese or religious order.
Christmas, Epiphany, and Corpus Christi had “privileged” octaves, during which certain highly ranked feasts might be celebrated.
Pope Pius XII further simplified the Calendar with a decree dated 23 March 1955: only the octaves of Christmas, Easter and Pentecost were kept, as these did not repeat the same liturgy daily.
"[12] Churches within the Anglican Communion traditionally observed octaves associated with the feasts of Christmas, Corpus Christi, Epiphany, Michaelmas, Easter, and All Saints' Day.
The octave was established in 1895 by Pope Leo XIII for the period between Ascension and Pentecost.
[15] In 1968, the World Council of Churches and the Vatican's Pontifical Council for Promoting Christian Unity agreed to jointly publish prayer materials for the occasion under the title "Week of Prayer for Christian Unity", but it is still often referred to as an octave, especially within the Roman Catholic, Lutheran and Anglo-Catholic traditions.