Query: What were revised between 1951 and 1955?
Context: During the pontificate of Pius XII, the Vatican eased regulations on the use of Latin in Roman Catholic liturgies, permitting some use of vernacular languages during baptisms, funerals and other events. In 1951 and 1955, the Easter liturgies underwent revision, most notably including the reintroduction of the Easter Triduum. The Second Vatican Council made no changes to the Roman Missal, but in the document Sacrosanctum Concilium mandated that a general revision of it take place.