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