The eighth day after a Festival. The intervening days are said to be "of" or within its Octave and partake of the character of the Festival. The only Feasts mentioned in the Prayer Book, having an Octave as of obligation are Christmas, Easter, Ascension and Whitsun Day, each being honored with a Proper Preface in the Communion Office which is to be used each day during the week. Trinity Sunday was formerly the Octave of Whitsun Day, and probably for this reason its Proper Preface is not repeated during the week.