The text below is taken from the General Instruction on the Liturgy of the Hours.
Christ taught us: “You must pray at all times and not lose heart”. The Church has been faithful in obeying this instruction; it never ceases to offer prayer. The Church fulfills this precept not only by celebrating the eucharist but in other ways also, especially through the liturgy of the hours. What distinguishes the liturgy of the hours from other liturgical services is that it consecrates to God the whole cycle of the day and the night. To the different hours of the day the liturgy of the hours extends the praise and thanksgiving, the memorial of the mysteries of salvation, the petitions and the foretaste of heavenly glory that are present in the eucharistic mystery. The liturgy of the hours is in turn an excellent preparation for the celebration of the eucharist itself, for it inspires and deepens in a fitting way the dispositions necessary for the fruitful celebration of the eucharist: faith, hope, love, devotion, and the spirit of self-denial.
Office of Readings
The office of readings seeks to provide God’s people, and in particular those consecrated to God in a special way, with a wider selection of passages from sacred Scripture for meditation, together with the finest excerpts from spiritual writers.
Morning prayer is intended to sanctify the morning. St. Basil the Great gives an excellent description of this character in these words: “It is said in the morning in order that the first stirrings of our mind and will may be consecrated to God and that we may take nothing in hand until we have been gladdened by the thought of God.
Following a very ancient tradition Christians have made a practice of praying out of private devotion at various times of the day, even in the course of their work. Liturgical custom in both East and West has retained midmorning, midday, and midafternoon prayer. It is permitted to choose from the three hours the one most appropriate to the time of day, so that the tradition of prayer in the course of the day’s work may be maintained.
When evening approaches and the day is already far spent, evening prayer is celebrated in order that “we may give thanks for what has been given us, or what we have done well, during the day.”
Night prayer is the last prayer of the day, said before retiring, even if that is after midnight.
If you’re getting started with the Liturgy of the Hours, I recommend you learn using the wonderful website at www.divineoffice.org. They have received approval from the US Council of Catholic Bishops to publish the text of the Liturgy of the Hours on their website, and they have organized everything beautifully. You can simply click on the day and hour you’d like to pray and pray with other Catholics around the world. They are worthy of your support.