Today I have noticed the amusing and ironic fact that in the modern form of the Roman Rite Mass, for which one of the abiding marks in popular consciousness is the priest and assembly facing each other, the celebrant is actually directed to face the people fewer times than in the Tridentine Liturgy!
So, in the reformed Roman Rite, for which Mass versus populum must be possible but is not required, the priest must face the people at six moments:
* When giving the opening greeting (GIRM 124);
* When giving the invitation to pray, "Orate, fratres" (GIRM 146);
* When giving the greeting of peace, "Pax Domini sit semper vobiscum" (GIRM 154);
* When displaying the consecrated Host (or Host and Chalice) before Communion and saying: "Ecce Agnus Dei" (GIRM 157);
* When inviting to pray ("Oremus") before the postcommunion prayer (GIRM 165);
* When giving the final blessing (Ordo Missae 141).
But in the Tridentine Mass, celebrated according to the 1962 Missale Romanum, the priest is required to face the people at eight moments:
* When greeting the people ("Dominus vobiscum") before the collect (Ritus servandus in celebratione Missae, V, 1);
* When greeting the people ("Dominus vobiscum") before the offertory rite (Ritus servandus, VII, 1);
* When giving the invitation to pray, "Orate, fratres" (Ritus servandus, VII, 7);
* Twice before giving Communion to others, first when saying the two prayers after the Confiteor, and again while displaying a consecrated Host and saying "Ecce Agnus Dei" (Ritus servandus, X, 6);
* When greeting the people ("Dominus vobiscum") before the postcommunion prayer (Ritus servandus, XI, 1);
* When saying "Ite, missa est" (Ritus servandus, XI, 1);
* When giving the last part of the final blessing (Ritus servandus, XII, 1).
Source: Wikipedia article "Versus Populum," used according to Creative Commons Attribution Licensing, which is what I also use on this blog. (Scroll all the way down.)