ChapterIV-III. Calendar and Option to Choose an Office or Part of an Office
Calendar to be Followed
241. The office in choir and in common is to be celebrated according to the proper calendar of the diocese, of the religious family, or of the individual churches.  Members of religious institutes join with the community of the local Church in celebrating the dedication of the cathedral and the feasts of the principal patrons of the place and of the wider geographical region in which they live. 
242. When clerics or religious who are obliged under any title to pray the divine office join in an office celebrated in common according to a calendar or rite different from their own, they fulfill their obligation in respect to the part of the office at which they are present.
243. In private celebration, the calendar of the place or the person's own calendar may be followed, except on proper solemnities and on proper feasts. 
Option to Choose an Office
244. On weekdays when an optional memorial is permitted, for a good reason the office of a saint listed on that day in the Roman Martyrology, or in an approved appendix to it, may be celebrated in the same way as other memorials (see nos. 234-239).
245. For a public cause or out of devotion, except on solemnities, the Sundays of the seasons of Advent, Lent, and Easter, Ash Wednesday, Holy Week, the octave of Easter, and 2 November, a votive office may be celebrated, in whole or in part: for example, on the occasion of a pilgrimage, a local feast, or the external solemnity of a saint.
Option to Choose Texts
246. In certain particular cases there is an option to choose texts different from those given for the day, provided there is no distortion of the general arrangement of each hour and the rules that follow are respected.
247. In the office for Sundays, solemnities, feasts of the Lord listed in the General Calendar, the weekdays of Lent and Holy Week, the days within the octaves of Easter and Christmas, and the weekdays from 17 to 24 December inclusive, it is never permissible to change the formularies that are proper or adapted to the celebration, such as antiphons, hymns, readings, responsories, prayers, and very often also the psalms.
In place of the Sunday psalms of the current week, there is an option to substitute the Sunday psalms of a different week, and, in the case of an office celebrated with a congregation, even other psalms especially chosen to lead the people step by step to an understanding of the psalms.
248. In the office of readings, the current cycle of sacred Scripture must always be respected. The Church's intent that "a more representative portion of the holy Scriptures will be read to the people in the course of a prescribed number of years"  applies also to the divine office.
Therefore the cycle of readings from Scripture that is provided in the office of readings must not be set aside during the seasons of Advent, Christmas, Lent, and Easter. During Ordinary Time, however, on a particular day or for a few days in succession, it is permissible, for a good reason, to choose readings from those provided on other days or even other biblical readings - for example, on the occasion of retreats, pastoral gatherings, prayers for Christian unity, or other such events.
249. When the continuous reading is interrupted because of a solemnity or feast or special celebration, it is allowed during the same week, taking into account the readings for the whole week, either to combine the parts omitted with others or to decide which of the texts are to be preferred.
250. The office of readings also offers the option to choose, with a good reason, another reading from the same season, taken from The Liturgy of the Hours or the optional lectionary (no. 161), in preference to the second reading appointed for the
day. On weekdays in Ordinary Time and, if it seems opportune, even in the seasons of Advent, Christmas, Lent, and Easter, the choice is open for a semicontinuous reading of the work of a Father of the Church, in harmony with the biblical and liturgical context.
251. The readings, prayers, songs, and intercessions appointed for the weekdays of a particular season may be used on other weekdays of the same season.
252. Everyone should be concerned to respect the complete cycle of the four-week psalter.  Still, for spiritual or pastoral advantage, the psalms appointed for a particular day may be replaced with others from the same hour of a different day. There are also circumstances occasionally arising when it is permissible to choose suitable psalms and other texts in the way done for a votive office.