Today is Ember Saturday in Advent and also the anticipated Vigil of St. Thomas the Apostle. It ranks as a greater, non-privileged ferial day, of simple rite.
Mattins has one nocturn and three lessons. The lessons are from a homily of St. Gregory on St. Luke's Gospel. After the third responsory the second scheme of Lauds is sung with the set of special antiphons Intuemini etc (this set of antiphons for the Saturday before Christmas Eve is quite modern in its introduction ot the Roman liturgy. Prior to 1911-13 on the Saturday before Christmas Eve the set that fell on the day of the week St. Thomas' feast were either anticipated or transferred to the Saturday). The ferial preces are sung whilst kneeling after the Benedictus and before the collect. The Vigil is not commemorated in the Office. At the Hours again the special set of antiphons are used in sequence and the ferial preces are sung, kneeling, at each Hour.
The Mass, sung after None, has the usual, ancient form, for Ember Saturdays. The ministers wear folded chasubles. Four candles are on the altar. After the Kyrie there are a series of five structural units comprising of the invitation Oremus, followed by Flectamus genua, Levate, a collect, O.T. reading and gradual. Four of these readings are from Isaiah and the last from Daniel. After the pericope from Daniel instead of a gradual the hymn of the Three Men in fiery furnace is sung, Benedictus es, Domine, Deus patrum nostrorum and its collect Deus, qui tribus pueris. After this collect the Vigil is commemorated and the third collect is of the BVM, Deus, qui de Beate. Mass then continues as usual (with of course kneeling for the orations and from the Canon through to the Fraction as usual on penitential days) with Benedicamus Domino as the dismissal and the last Gospel of the Vigil.
A private Mass may be of the Vigil with the second collect of the Ember Saturday, the third collect of the BVM, Deus, qui de Beate. The same praxis about kneeling is followed and Benedicamus Domino sung and the last Gospel of the Ember Saturday.
Vespers of the fourth Sunday in Advent are sung. The Great 'O' Antiphon today is O radix Jesse which is both 'doubled' and sung entire both before and after the canticle. At compline the preces are sung, standing.
Following the 'liturgical books of 1962' the Canticle of Moses at Lauds shortened from 65 to 27 verses. The Vigil of St. Thomas has been abolished. At Prime the fourth psalm is not added and the ferial chapter Pacem et veritatem is replaced by the festal (!) Regi saeculorum. The ferial preces are omitted at Prime and the Hours. The Mass has the option of the 'mini-Ember Day' consisting of just one additional unit of collect, pericope and gradual. There is no commemoration of the Vigil or prayer to the BMV, the dismissal is Ite, missa est and the ministers wear dalmatic (the garment of joy!) and tunicle.
One strange feature of 1962 Ember Wednesdays and Fridays is that whilst traditionally the Ember Day liturgy ended with Mass sung after None in the 1962 books Vespers becomes formally part of the Ember Day. Hitherto, if Vespers were ferial, the collect from the preceding Sunday was sung. In 1962 use the collect from the Ember Day is used at Vespers. (In Lent of course the collects are proper to each feria).
Earlier in the week the learned Dr. Michael Brown of the Forest Murmurs blog raised an interesting question of whether the precedence given to the Ember Days in the 1962 rite was preferable. It is a good question and I only replied briefly in the comments whilst in Ireland. Considering the Office, the Ember Day's lessons can all be read as the ninth lesson in the Office of an occurring feast and so are not lost. Their responsories are not said but are used in the ferial Office of Advent anyway. The characteristic preces of penitential days are not sung at the Little Hours in the 1962 rite and Prime loses a psalm, chapter and preces. The Rorate Mass for Ember Wednesday (see the excellent article by Gregory DiPippo here) was actually used in many places on other days in Advent. It was even used on Sundays (!) by a decision of the SCR of 1713 for a special novena in Catania, and given a Credo for the Friars Minor in 1718. In Poland by the middle of the eighteenth century there was a solemn daily celebration of the Rorate Mass often sung before dawn. According to general liturgical law the Rorate Mass is the Votive Mass of the BVM to be used in Advent and can be celebrated in preference to the ferial Mass most days of Advent anyway. On the Ember Wednesday itself in parish churches either the Mass of the Ember Day or the feast could be sung. 'Private' Mass in this context does not necessarily mean a read Mass. With the 1962 Ember Wednesday Mass there are of course all the usual changes associated with the 1962 Ordo Missae: no bows to the Cross whilst at the sides of the altar, the vestments of the ministers on penitential days, the reduced period of kneeling on penitential days, the celebrant not reading the Epistle and Gospel but reading the Gradual etc (which is highly inconsistent), the new rules for the level of the celebrant's voice, the loss of Benedicamus Domino etc. With the not insignificant changes to the Office and Mass found in the 1962 books and the flexibility allowed by the more traditional praxis I still maintain the older use both preferable and, in the Collegiate use, ideal.