CELEBRET. – A certificate issued by the competent ecclesiastical authority attesting that a priest is authorized to celebrate Mass. It contains the name of the holder, the date, the time limit, and the diocesan or institute’s seal with the signature of the Ordinary if the priest is secular, or of the superior if he is religious. For a priest of the Eastern rite, a certificate from the Sacred Congregation for the Eastern Churches suffices.
There are two cases in which a parish priest or the rector of a church may authorize a passing priest to say Mass without a CELEBRET:
1) when the priest is known to him as a person of unquestionable integrity;
2) if the priest is unknown, he may permit the celebration once or twice only, provided that the priest wears ecclesiastical dress, does not accept remuneration, and signs a special register indicating his office and the diocese to which he belongs (can. 804).