By the way, can anyone tell me what an octave is in this context? Thanks.
Generally, an octave is any group or series of eight. The term has specific use in music and poetry, of course. In church usage, it's a period of eight days, usually beginning with an important feastday. In the current Roman calendar, only Christmas and Easter have octaves: the celebration of the feastday extends through the next seven days (eight, counting the feastday itself). January 18 through January 25 was originally chosen as the Octave of Prayer for Christian Unity because the Catholic Church and the Church of England both celebrated the Feast of the Chair of St. Peter on January 18 and the Feast of the Conversion of St. Paul on Jan. 25. After Vatican II, St. Peter's feast in the Roman calendar was restored to February 22, where it had been originally, though Anglicans still celebrate it on Jan. 18.