Is there an obligation to repeat the Hours in the order fixed in the Breviary? Yes, there is such an obligation. And a person may sin venially by the inversion of the Hours, The obligation binds sub veniali only. The inversion does not mean any grave breach of order, which is fixed by a secondary precept and as a circumstance of light importance. If the whole office be recited, the substance of the office—which is the main and primary matter—is safeguarded. Several authors argued that any inversion of the Canonical Hours, if frequent, is a mortal sin, but the opinion which says that the inversion of the Hours is only a venial sin is the more probable (St. Alph. 169; Gury, 77; Lehmkuhl II., 621).
Which causes justify an inversion of the Hours? Any reasonable cause justifies this inversion. Thus, if a friend invite a priest to joint recitation of an Hour, and the priest have not the preceding canonical Hours recited, he is justified in accepting the invitation and in inverting the order of the Hours. Or if a person have a Diurnal only at hand, he may read the day Hours, although he have not Matins for the day read. Again, a priest may not have the lessons for Matins at hand, but he may recite the psalms for Matins, Lauds, and add the lessons at Matins when they are to hand (Gury, n. 78; St. Alph., n. 170).
Is it a sin to say Matins for following day before finishing office of current day? Some theologians answer affirmatively, because the office of the current day should be complete before another office is begun. Others hold that such recitation is both valid and licit, as the office of one day and its obligation have no bond with the office of another day, and that any reasonable cause exempts from all sin or fault (Gury, n. 79). Not to recite the commemorations in the prescribed order set out in the Ordo is held by some theologians to be a venial sin, as they hold that the rubric is preceptive; others hold that it is not any sin, as they say that the rubric is directive.
The time fixed for the recitation of the entire office of the day is from midnight to the midnight following, and anyone bound to recite the Divine Office does not sin gravely if he has recited carefully the entire office of the day between these limits of time; because, within these limits, the substance of the obligation binding to time is fulfilled. Of course, it is lawful in virtue of a privilege granted by the Church to recite on the previous evening Matins and Lauds for the following day. In the recitation the times fixed by the Church for each hour should be observed. But the non-recital at those fixed times is never a mortal sin and is rarely a venial sin, unless their postponement or anticipation is without cause.