office was suppressed for the rest of the Church, and the office of the Nativity of the Blessed Virgin was substituted instead, the word "Conception" being substituted for "Nativity". According to the Papal Bull Commissi Nobis Divinitus, dated 6 December 1708, Pope Clement XI mandated the feast as a Holy Day of oligation which is to be celebrated in future years by the faithful. Furthermore, the pontiff requested that the papal bull be notarized in the Holy See to be further copied and reproduced for dissemination. Prior to Pope Pius IX's definition of the Immaculate Conception as a Roman Catholic dogma in 1854, most missals referred to it as the Feast of the Conception of the Blessed Virgin Mary. The festal texts of this period focused more on the action of her conception than on the theological question of her preservation from original sin. A missal published in England in 1806 indicates the same Collect for the feast of the Nativity of the Blessed Virgin Mary was used for this feast as well. The first move towards describing Mary's conception as "immaculate" came in the 11th century. In the 15th century, Pope Sixtus IV, while promoting the festival, explicitly tolerated both the views of those who promoted it as the Immaculate Conception and those who challenged such a description, a position later endorsed by the Council of Trent. The proper for the feast of the Conception of the Blessed Virgin Mary in the Medieval Sarum Missal merely addresses the fact of her conception. The collect for the feast reads: O God, mercifully hear the supplication of thy servants who are assembled together on the Conception of the Virgin Mother of God, may at her intercession be delivered by Thee from dangers which beset us. Roman Catholic Church Pope Pius IX in 8 December 1854 issued the Apostolic constitution Ineffabilis Deus: According to the Universal Norms on the Liturgical Year and the Calendar, the solemnity of the Immaculate Conception cannot replace an Advent Sunday; if 8 December falls on a Sunday, the solemnity is transferred to the following Monday. (In some countries, including the United States, the obligation to attend Mass does not transfer.) The 1960 Code of Rubrics, still observed by some in accordance with Summorum Pontificum, gives the feast of the Immaculate Conception preference even over an Advent Sunday. Anglican Communion In the Church of England, the "Conception of