Ash Wednesday is a privileged greater feria although of simple rite. No feast can be celebrated on this day of whatever rank. The liturgical colour of the day is violet.
The third part of the Pontificale Romanum still contains the rite for the Expulsion of Public Penitents - De Expulsione Publice Poenitentium ab Ecclesia in feria Quarta Cinerum. In that rite, after Sext, when the bishop was to celebrate Mass this day, the bishop vested in pontificals as far as the dalmatic and then donned a violet cope. Public penitents assembled in sack-cloth in their bare feet and were given a penance for Lent and then were given ashes, which the bishop had blessed. Then followed a ceremony that included the Seven Penitential Psalms and Litany. It culminated in the penitents being led, physically, from the church and expelled. The Caermoniale Episcoporum alludes to the Expulsion of Penitents in churches where it is the custom, C.E. Lib II, Cap.18, 2. Although no longer generally practised echoes of the above can be found in today's Liturgy which contains a general imposition of Ashes. The Liturgy is interesting too in that much remains of the Office of the Septuagesima season as originally Lent began with the Office of the first Sunday.
The Expulsion of Penitents from a 1611 edition of the Pontificale Romanum from Google Books.
The Office of Ash Wednesday begins with Mattins. The invitatory is Deum magnum Dominum, Venite adoremus and the Office hymn is Rerum Creator optime both from the ferial Office per annum. The antiphons Speciosus forma etc are sung with the nine psalms/psalm divisi of a single nocturn from the ferial Office for Wednesdays. The second scheme of Mattins for Wednesday is used, wherein Psalm 49 is split into three divisi rather than the usual two. The lessons in the nocturn are from a homily of St. Augustine reflecting on the LORD's Sermon on the Mount from St. Matthew's Gospel. The responsories are taken from the third nocturn of Quinquagesima Sunday. At Lauds the second scheme of psalmody is used (Pss. 50, 64, 100, Canticle of Anna & 145. The antiphon on the Benedictus and collect are proper to the day. After the antiphon on the Benedictus has been sung in full after the canticle the choir kneels and the ferial preces are sung. After the collect of the day commemorations are sung of the simplified feast of St. Peter's Chair at Antioch and of St. Paul the Apostle.
At the Hours again the ferial antiphons and psalter are used. Prime has a fourth psalm displaced from Lauds by the Miserere, so the psalmody at Prime is Pss. 25, 51, 52 & 96. At Prime both the Dominical and ferial preces are sung kneeling, the chapter is the ferial Pacem et veritatem. At the other Hours the short set of ferial preces are chanted with the choir again kneeling.
The blessing of Ashes and Mass follows None. The celebrant vests in a violet cope while the deacon and subdeacon wear violet folded chasubles. The organ is silent following the general rule when folded chasubles are worn. The Ashes, made from burning of the preceding year's Palms, are blessed with four collects of blessing. The short conclusion of these prayers may indicate the blessing was separated by the Expusion of Penitents from the Mass which follows. The Ashes are imposed whilst two antiphons are chanted Immutemur habitu and Inter vestibulum. These are followed by the poignant responsory Emendemus in melius. The Mass that now immediately follows is proper. The Gloria is not sung. The second collect is of St. Peter, the third collect of St. Paul. The preface of Lent is sung for the first time this year. The ferial tones are used for the orations, preface and Pater noster. The choir kneels for the Orations and from the Sanctus until Pax Domini. After the last collect the deacon sings Humiliate capita vestra Deo and the celebrant sings the Oratio super populum. The dismissal is Benedicamus Domino, sung by the deacon facing the celebrant and altar. The last Gospel is from the Mass of St. Peter's Chair.
Vespers are first Vespers of the following feast of St. Peter Damian with a corresponding colour change to white. Commemorations are sung of the feria (the antiphon on the Magnificat and collect are proper) and of St. Peter and of St. Paul.
In the 'liturgical books of 1962' the ferial preces are sung at Lauds and Vespers only. At Lauds there are no commemorations. At Prime there are three psalms 25, 51 & 52 and the chapter is the festal Regi saeculorum. At the blessing of Ashes and Mass the ministers wear dalmatic and tunicle. Judica me Deus etc is omitted from the beginning of Mass. There is only one collect. The extended kneeling is omitted with the choir rising at the conclusion of the Canon. The dismissal is Ite missa est. Vespers are of the feria without any commemorations.