The apostolic constitution, given on Holy Thursday, by which Paul VI promulgated the Roman Missal revised by decree of the Second Vatican Council. Recalling how Saint Pius V had published the Missal of 1570 after the Council of Trent, the Pope explains the principal changes: a revised Order of Mass in which the rites are simplified and the parts proper to each minister and to the assembly are made clear; the addition of three new Eucharistic Prayers to the Roman Canon, which is retained; a great enrichment of the prefaces; the restoration of the prayer of the faithful; proper prayers for the ferias of Advent, Christmas, Lent and Easter; and a far wider selection of Scripture spread over a multi-year cycle so that the table of God's word may be more lavishly prepared. He gives these prescriptions the force of law and sets them to take effect on the First Sunday of Advent, 30 November 1969.