Part I: Renewal in Accord with Tradition
3. In response to the requests of the fathers of the Council of Trent, concerned with the reform of the Church in their time, Pope St. Pius V saw to the reform of the liturgical books, above all the breviary and the missal. It was toward this same goal that the succeeding Roman pontiffs directed their energies during the subsequent centuries in order to ensure that the rites and liturgical books were brought up to date and when necessary clarified. From the beginning of this century they undertook a more general reform.
Pope St. Pius X established a special commission for this reform, and he thought that it would take a number of years for it to complete its work; however, he laid the foundation stone of this edifice by renewing the celebration of Sunday and by reforming the Roman breviary.  "In fact this all demands," he affirmed, "according to the views of the experts, a work both detailed and extensive; and therefore it is necessary that many years should pass before this liturgical edifice, so to speak, . . . reappears in new splendor in its dignity and harmony, once the marks of old age have been cleared away." 
Pope Pius XII took up again the great project of liturgical reform by issuing the encyclical Mediator Dei  and by establishing a new commission.  He likewise decided important matters, for example: authorizing a new version of the Psalter to facilitate the understanding of the psalms;  the modification of the eucharistic fast in order to facilitate access to holy communion; the use of contemporary language in the ritual; and, above all, the reform of the Easter vigil  and Holy Week. 
The introduction of the Roman Missal of 1963 was preceded by the declaration of Pope John XXIII, according to which "the fundamental principles related to the general reform of the liturgy were to be entrusted to the fathers in the forthcoming ecumenical council." 
4. Such an overall reform of the liturgy was in harmony with the general hope of the whole Church. In fact, the liturgical spirit had become more and more widespread, together with the desire for an "active participation in the most holy mysteries and in the public and solemn prayer of the Church,"  and a wish to hear the word of God in more abundant measure. Together with the biblical renewal, the ecumenical movement, the missionary impetus and ecclesiological research, the reform of the liturgy was to contribute to the overall renewal of the Church. I drew attention to this in the letter Dominicae Cenae: "A very close and organic bond exists between the renewal of the liturgy and the renewal of the whole life of the Church. The Church not only acts, but also expresses herself in the liturgy and draws from the liturgy the strength for her life." 
The reform of the rites and the liturgical books was undertaken immediately after the promulgation of the constitution Sacrosanctum Concilium and was brought to an effective conclusion in a few years, thanks to the considerable and selfless work of a large number of experts and bishops from all parts of the world. 
This work was undertaken in accordance with the conciliar principles of fidelity to tradition and openness to legitimate development,  and so it is possible to say that the reform of the liturgy is strictly traditional and in accordance with "the ancient usage of the holy fathers."