While the liturgical reforms promulgated by the Second Vatican Council were unexpected by many people, the seeds of the reform actually were planted centuries earlier at the Council of Trent (1545-1563).
The Council of Trent was a monumental moment for the Church. Held in the decades following the Reformation, the council sought to define, unite, and codify many of the Church’s dogmas and practices. In terms of the liturgy, the most significant task that they accomplished was directing the pope to standardize and revise the books for the celebration of Mass.