It is asked whether a person fulfils his debt to the Church if he has recited by mistake an office other than the one assigned in the calendar of the day.  Theologians teach that such a recitation fulfils the debt.  The Church does not wish to impose a second recitation, and her axiom “officium pro officio valet” holds, provided always that the order of the psalms as laid down in the new psaltery is followed.  This order is necessary always for validity.  However, if the substituted office be very much shorter than the omitted office, it is advised to equalise them by reciting the psalms of Matins, This is a counsel and was not laid down by theologians as an obligation.

An office thus omitted is not to be transferred to another day (S.C.R., June 17th, 1673).  The office may be omitted altogether for that year.  If there be leisure the omitted office should be recited.  This practice is in conformity with the spirit of the liturgy and with the right order of the calendar.  The Sacred Congregation of Rites, questioned on this matter, replied sic debere fieri, such should be done.  If a priest recites by mistake one day’s office for another (e.g., the Tuesday office on a Monday) he is bound to recite Tuesday’s office on Tuesday (St. Alphonsus).  If, however, after a portion of the office has been read, it is noticed that a mistake has been made in reading the calendar or the Ordo, and that the office partly recited is not the office of the current day, what is to be done?  If the priest has without fault made the mistake of reciting some office not ascribed to the current day, he is not bound to repeat the part already recited (e.g., Matins); it is sufficient, valid and lawful to follow the correct office in the following Hours.  The priest reciting is not bound to repeat even part of an hour, if he finds out his mistake during the recitation of even a small hour.  And he may finish the psalm or hymn or prayer which he was reciting when he discovered his mistake, and he may then take up the correct office at the part or hour at which he leaves off, or he may finish the Hour at which he was engaged.  The former solution of the difficulty seems the better, as it more accurately agrees with the maxim, error corrigatur ubi apprehenditur.  If the error in the selecting of the office has been wilful, say, through gross carelessness, and is the fault of the priest who changes a notable part of a canonical Hour, he is obliged—­the more probable opinion teaches—­to repeat the full Hour, and this obligation binds under pain of venial sin—­i.e., the obligation to recite the office in the prescribed manner.

What is a person bound to do who forgets part of an Hour—­is he obliged to repeat the full Hour?

He is bound to recite the part forgotten only, unless the mistake be made through gross carelessness, and unless it be a considerable part (e.g., two nocturns); in that case he is bound under pain of venial sin to repeat the full Hour.  If a person say the same Hour (e.g., Terce) twice, may he compensate for extra labour by the omission of an equivalent part (e.g., None)?  Such omission is unlawful; he must recite all the Hours without omission (Scavini, 391).