147 The Psalms, as all know, form the chief part of the divine office. They encompass the full round of the day and sanctify it. Cassiodorus speaks beautifully about the Psalms as distributed in his day throughout the divine office: "With the celebration of matins they bring a blessing on the coming day, they set aside for us the first hour and consecrate the third hour of the day, they gladden the sixth hour with the breaking of bread, at the ninth they terminate our fast, they bring the evening to a close and at nightfall they shield our minds from darkness."
Source: Mediator Dei (Vatican.va)