1. On Sundays of Advent, the Gloria is omitted. On ferias when the Mass of the preceding Sunday is said, the Alleluia and its verse are omitted. The faithful should kneel for the Collects and Postcommunion prayers on the ferias of Advent (not, however, on Sundays, feasts days, nor on the Vigil of Christmas).
2. Commemorations of the feria in Advent are privileged and must be made on feasts of Saints at every Mass as well as Lauds and Vespers.
3. On Sundays and ferias of Advent (except the Third Sunday), the altar may not be ornamented with flowers, and the organ is only used to sustain the choir, and only then if necessary.
4. On the Third Sunday of Advent ("Gaudete Sunday"), rose vestments are preferred but violet vestments may be worn. Organ and flowers are permitted.
5. In place of the Preface of the Blessed Trinity on Sunday and the Common Preface on weekdays, the Preface for Advent may be used.
6. From December 17th to December 23rd, inclusive, the Antiphon for the Magnificat at Vespers is one of the "O" Antiphons [see following posts].
7. From Saturday November 29th to February 1st inclusive, the final Antiphon of the Blessed Virgin Mary at Compline is Alma Redemptoris Mater.
(excerpted/adapted from the 2009 Liturgical Ordo, Priestly Fraternity of St Peter)