ChapterII-I. Introduction to the Whole Office
34. The whole office begins as a rule with an invitatory. This consists in the verse, Lord, open my lips. And my mouth will proclaim your praise, and Ps 95. This psalm invites the faithful each day to sing God's praise and to listen to his voice and draws them to hope for "the Lord's rest." 
In place of Ps 95, Ps 100, Ps 67, or Ps 24 may be used as circumstances may suggest.
It is preferable to recite the invitatory psalm responsorially as it is set out in the text, that is, with the antiphon recited at the beginning, then repeated, and repeated again after each strophe.
35. The invitatory is placed at the beginning of the whole sequence of the day's prayer, that is, it precedes either morning prayer or the office of readings, whichever of these liturgical rites begins the day. The invitatory psalm with its antiphon may be omitted, however, when the invitatory is the prelude to morning prayer.
36. The variation of the invitatory antiphon, to suit the different liturgical days, is indicated at its place of occurrence.