"Octave" has its linguistic roots in the Latin word for eight, octo. Musicians speak of octaves when they refer to the eight note distance between matching tones. In the Church's liturgical calendar an octave is a way of making an especially important liturgical celebration last for a complete eight day cycle, beginning and ending on the same day of the week.
Throughout history, the Church had several octave feasts, but since reforms in the 1950's, it celebrates Christmas and Easter as octaves along with the "great octave" of the eight weeks between Easter and Pentecost.
