OCTAVE. The octave is the eighth day  after any principal festival of the Church. In ancient times it was customary to observe these days with much devotion, including the whole period also from the festival to the octave. In our Prayer Book we observe the octaves of Christmas, Easter, Ascension, and Whit-Sunday, by using the special preface appointed in the Communion Service at every celebration during the octave. The Whit-Sunday preface, however, is only used six days, because Trinity Sunday falls on the octave.