ChapterIII-IV. Plan for the Distribution of the Psalms in the Office
126. The psalms are distributed over a four-week cycle in such a way that very few psalms are omitted, while some, traditionally more important, occur more frequently than others; morning prayer and evening prayer as well as night prayer have been assigned psalms appropriate to these hours. 
127. Since morning prayer and evening prayer are particularly designed for celebration with a congregation, the psalms chosen for them are those more suited to this purpose.
128. For night prayer the norm given in no. 88 has been followed.
129. For Sunday, including its office of readings and daytime prayer, the psalms chosen are those that tradition has particularly singled out as expressions of the paschal mystery. Certain psalms of a penitential character or connected with the passion are assigned to Friday.
130. Three psalms (78, 105, and 106) are reserved for the seasons of Advent, Christmas, Lent, and Easter, because they throw a special light on the Old Testament history of salvation as the forerunner of its fulfillment in the New.
131. Three psalms (58, 83, and 109) have been omitted from the psalter cycle because of their curses; in the same way, some verses have been omitted from certain psalms, as noted at the head of each. The reason for the omission is a certain psychological difficulty, even though the psalms of imprecation are in fact used as prayer in the New Testament, for example, Rv 6:10, and in no sense to encourage the use of curses.
132. Psalms too long to be included in one hour of the office are assigned to the same hour on different days so that they may be recited in full by those who do not usually say other hours. Thus Ps 119 is divided in keeping with its own internal structure and is spread over twenty-two days during daytime prayer, because tradition has assigned it to the day hours.
133. The four-week cycle of the psalter is coordinated with the liturgical year in such a way that on the First Sunday of Advent, the First Sunday in Ordinary Time,
the First Sunday of Lent, and Easter Sunday the cycle is always begun again with Week I (others being omitted when necessary).
After Pentecost, when the psalter cycle follows the series of weeks in Ordinary
Time, it begins with the week indicated in the Proper of Seasons at the beginning of
the appropriate week in Ordinary Time.
134. On solemnities and feasts, during the Easter triduum, and on the days within the octaves of Easter and Christmas, proper psalms are assigned to the office of readings from those with a tradition of use at these times and their relevance is generally highlighted by the choice of antiphon. This is also the case at daytime prayer on certain solemnities of the Lord and during the octave of Easter. At morning prayer the psalms and canticle are taken from the Sunday of the Week I of the psalter. On solemnities the psalms at evening prayer I are taken from the Laudate Psalms, following an ancient custom. At evening prayer 11 on solemnities and at evening prayer on feasts the psalms and canticle are proper. At daytime prayer on solemnities (except those already mentioned and those falling on Sunday) the psalms are taken from the Gradual Psalms; at daytime prayer on feasts the psalms are those of the current week and day of the psalter.
135. In all other cases the psalms are taken from the current week and day of the psalter, unless there are proper antiphons or proper psalms.