Reform of the liturgy had been a part of the liturgical movements in the 20th century mainly in France, and Germany which were officially recognized by Pius XII in his encyclical Mediator Dei. 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. After the Vatican Council, in April 1969, Paul VI approved the "new Order of Mass" promulgated in 1970, as stated in the Acta Apostolica Sedis to "end experimentation" with the Mass and which included the introduction of three new Eucharistic Prayers to what was up to then a single Roman Canon.
What was re instated to the Easter liturgy by reform?
Easter Triduum