Today is the Ember Saturday in Advent. It ranks as a greater, non-privileged ferial day, of simple rite. Today also being the 17th of December sees, in the Roman rite, the start of the series of 'Great O' Antiphons and a series of special antiphons at Lauds and the Hours on the ferial days before the Vigil of the Nativity.
At Mattins the invitatory Prope est jam Dominus etc is sung and the hymn is Verbum supernum prodiens. In the nocturn the antiphons Memor fuit etc are sung with the nine psalms' divisi for Saturday. The three lessons are from a homily of St. Gregory on St. Luke's Gospel. After the third responsory, Veni, Domine, et noli tardare etc., the second scheme of Lauds for Saturday is sung. The special set of antiphons for the Saturday before the Vigil of the Nativity, Intuemini etc., are sung with psalms 50, 91, 63, the Canticle of Moses & 150. This set of antiphons for the Saturday before Christmas Eve is actually rather a modern addition to the series of antiphons at Lauds and the Hours that are used from the 17th December. Prior to the 1911-13 reform on the Saturday before Christmas Eve the particular set of antiphons that fell on the day of the week occupied by St. Thomas' feast were either anticipated or transferred to this Saturday. The hymn at Lauds is En clara vox. The antiphon on the Benedictus is Quomodo fiet istud etc. After the antiphon is sung in full after the canticle the choir kneels and the ferial preces are sung. The collect is proper to the Ember Saturday, Deus qui conspicis.
At the Hours the antiphons Intuemini etc are used in the usual sequence. At Prime the fourth psalm is added (the one displaced by the Miserere in the schema of Lauds II, Ps. 149) and the chapter is the ferial Pacem et veritatem. The Dominical and ferial preces are sung with the choir kneeling. At the other Hours the short set of ferial preces are sung, again with the choir kneeling.
Mass is sung after None and has the usual, ancient, form, common to 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 second collect is of the BVM, Deus, qui de Beate and the third collect for the Church, Ecclesiae, or Deus omnium. 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 sung as the dismissal by the deacon facing the altar.
Vespers of the fourth Sunday in Advent are sung. The antiphons Canite tuba etc are sung with the Saturday psalms. The Office hymn is Creator alme siderum and the Great 'O' Antiphon O Sapientiais sung. The 'Great O' Antiphons are 'doubled', i.e. they are sung entire both before and after the Magnificat even on days of simple rank. At Compline the Dominical preces are sung, with the choir standing.
Following the 'liturgical books of 1962' the Canticle of Moses at Lauds gets shortened from 65 to 27 verses. 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 are no additional collects for the Blessed Virgin or for the Church. The dismissal is Ite, missa est and the ministers wear dalmatic and tunicle. The doubling of the Great 'O' Antiphons is no different to all other antiphons throughout the year. The Dominical preces are omitted at Compline.