Since 2007, use of the Roman Missal of 1962 has been broadly permitted within the church. This creates, in effect, two liturgical years running concurrently. In Advent to Pentecost, Abbot Patrick Regan compares the prayers and prefaces, readings and rubrics, calendar and chants of the 1962 Missal with that of the Missal as it was revised following the Second Vatican Council, now in its third edition. The result is a striking demonstration of the splendor and superiority of the reformed Missal over its predecessor, at least as far as the liturgical year is concerned.Regans chapters on Advent, Lent, and the Easter Season are particularly informative because these seasons are so different in the two missals. Perhaps less obvious are the differences between Holy Week and the Triduum. Regan not only describes external modifications in the services as restored by Pius XII in 1956 but explores deeper theological currents, especially in the relationship between the passion and resurrection of the Lord in the one paschal mystery, to show how advances in this area find expression in the current Triduum celebrations and throughout the fifty days of Easter. The originality of the book lies mainly here. The most urgent liturgical challenge today, the author contends, is to raise the ars celebrandi to the same level of excellence as the Missal itself.