5.E.3 Proper Place of the Celebration of Mass
1. A priest may celebrate Mass in a decent place, outside of a sacred place, if the Mass had been arranged so that the faithful could fulfill an obligation to participate in Mass and the present church facilities or Mass schedule could not accommodate this.  (See notes on the Proper Place of the Celebration which follows.)
2. On other occasions, a priest may celebrate Mass for a parish or a group of the faithful outdoors or in a suitably arranged facility that is not a sacred place if the size of the assembly demands it or access to a sacred place cannot be arranged properly. (Illustratively, parish, vicariate or regional missions, retreats, parish-sponsored camp-outs, annual parish festival, migrant camps.)
3. A priest may celebrate Mass for the residents and staff of an institution for whom he provides pastoral care in a suitably arranged public room or “interdenominational” chapel of the institution. (The situations might include a hospital, a migrant camp, a convalescent home, or a prison.)
4. A priest may celebrate Mass in a private home or hospital room in which the Sacrament of the Anointing of the Sick is administered if he judges that a pastoral necessity exists.
5. The express permission of the local ordinary is required to celebrate the Eucharist in a place of worship of a non-Catholic Church or ecclesial community. Scandal is to be avoided. 
6. However, it is never lawful for a priest to celebrate Mass in a temple or sacred place of any non-Christian religion. 
 C. 1247; m.p. Pastorale Munus (December 3, 1963) n. 7; c. 932. See Congregation for Divine Worship and Discipline of the Sacraments, Instruction on Certain Matters to be Observed or to be Avoided Regarding the Most Holy Eucharist, Redemptionis Sacramentum (April 23, 2004) n. 108. Available at the Vatican website, http://www.vatican.va/roman_curia/congregations/ccdds/documents/rc_con_ccdds_doc_20040423_redemptionis-sacramentum_en.html. [Hereafter Redemptionis Sacramentum.]
 C. 933.
 Redemptionis Sacramentum, n. 109.