Occurrence is the coinciding or occurring of two liturgical offices on one and the same day; concurrence is the succession of two offices, so that the second vespers of one occur at the same time as the first vespers of the other. The chief causes of occurrence are: (1) the variableness of the feast and cycle of Easter, while the other feasts are fixed; (2) the annual change of the Dominical Letter, whereby Sunday falls successively on different dates of the same month (see CALENDAR; DOMINICAL LETTER). Occurrence may be accidental or perpetual.

(1) The calendar gives as a fixed feast for 28 May the feast of St. Augustine of Canterbury; on the other hand on 28 May, 1891, the table of movable feasts marked that day as the feast of Corpus Christi; thus on 28 May, 1891, these two offices fell on the same day - that is there was an occurrence. But as this coincidence was due to a variable cause, and did not happen the following years, the occurrence was accidental.
(2) The patronal feast of churches is celebrated with an octave; in the case of a church having St. Martin (11 November) as its patron, the octave day (18 November) falls on a fixed feast marked in the Calendar: "Dedication, etc . . ."; consequently, there is in such a church each year a coinciding of two offices on 18 November; this occurrence is said to be perpetual.

II. RULES TO BE OBSERVED

In case of an occurrence two questions arise: (1) Which office is to have the preference? (2) What is to be done concerning the less favoured office?

(1) The two offices must be compared from the point of view of dignity and of necessity, taken either separately or together. As to dignity, Christmas, the Assumption, etc., prevail over the feasts of saints; as to necessity, the first Sunday of Advent being privileged prevails (if it falls on 30 November) over the Office of St. Andrew the Apostle; a fortiori, an office favoured by both conditions will be preferred.
(2) As to the less favoured office, it is treated differently according as the recurrence is perpetual or accidental. If perpetual, the authority of the Holy See should intervene to operate a change that will be effectual each year; the mention of the feast is maintained on the day on which it falls, but the office is changed to the first free day (a day not occupied by another office, double or semi-double); liturgists call this change mutatio (not translatio). When the occurrence is accidental, the compiler of the diocesan ordo, with the approval of the ordinary, decides, in conformity with the rubrics, what is to be done for the year. Either the office in question is transferable, in which the regulations of title X, "De translatione", are to be followed; or else it is not transferable, when it must be seen if it is to be omitted completely, or if a commemoration of it may be made on the day in question. The whole matter is provided for in the general rubrics of the Breviary.

To give an instance of concurrence, the ecclesiastical calendar marks the feast of St. Anthony of Padua on 13 June, and that of St. Basil on 14 June; these two feasts being of double rite have first and second Vespers; on the evening of 13 June, therefore, the second Vespers of St. Anthony and the first Vespers of St. Basil happen at the same time, and there is said to be a concurrence of the two offices.

GAVANTI, Thesaurus sacr. rit. cum additionibus Merati (3 vols., Venice, 1769); GUYETUS, Heortologia (Urbini, 1657); MENGHINI, Elementa juris liturg. (Rome, 1907); VAN DER STAPPEN, Tractalus de offic. div. (Mechlin, 1898)

Fernand Cabrol.