How the antiphons were chosen:
116. Proper antiphons are given for each of the psalms of morning prayer and evening prayer during the Easter triduum, on the days within the octaves of Easter and Christmas, on the Sundays of the seasons of Advent, Christmas, Lent, and Easter, on the weekdays of Holy Week and the Easter season, and from the 17th to the 24th of December.
117. On solemnities proper antiphons are given for the office of readings, morning prayer, the daytime hours, and evening prayer; if not, the antiphons are taken from the common. On feasts the same applies to the office of readings and to morning prayer and evening prayer.
118. Any memorials of the saints that have proper antiphons retain them (see no. 235).
119. The antiphons for the Canticles of Zechariah and of Mary are taken, during Ordinary Time, from the Proper of Seasons, if they are given there; if not, they are taken from the current week and day of the psalter. On solemnities and feasts they are taken from the proper if they are given there; if not, they are taken from the common. On memorials without proper antiphons the antiphon may be taken at will either from the common or from the current week.
120. During the Easter season Alleluia is added to all antiphons, unless it would clash with the meaning of a particular antiphon.