What is the ordo? This is a small book, published annually by the Archdiocese of Birmingham, that tells the priest what Mass is to be celebrated on a particular day. The word “Catholic” as used in ‘The Catholic Church’ means “universal”. The main way that the Church expresses its universality is through the Liturgy of the Mass and the ordo, and similar books published by dioceses around the world, ensure that the same Mass is celebrated in every Catholic church and chapel throughout the world. Basically - go into any Roman Catholic church anywhere in the world on the twenty-third Sunday in Ordinary Time in whichever of the three year cycle of readings is being proclaimed and the same Mass will be celebrated.