is not, if there be a reasonable cause for the change. For instance, if
a priest cannot say the office proper to his diocese on a certain day,
but says some other approved office, the change is not a sin. But if a
priest, _ex industria_, substitute one office for another, it is _per
se_ a venial sin; but if an office be said which is very much shorter
than the calendar office, or if this changing or substituting be so
frequent as to disturb gravely the good order of the year's offices, the
sin may be (and, according to some authors, is) a mortal sin.

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