In the Ordinary Form of the Roman Rite, there are four Sequences which in the Third Missal, follow the Alleluia, prior to that, they preceded it as in the Extraordinary Form. Two are mandatory, Victimae Paschalis of Easter and Veni, Sancte Spiritus of Pentecost and yes, I said, mandatory. Two are, sadly, optional; these are Lauda, Sion Salvatorem of the Solemnity of Corpus Christi and Stabat Mater on Our Lady of Sorrows, but not to the common tune that is sung as a hymn. In the Extraordinary Form they are not optional and there is one additional, Dies Irae in the Requiem Mass and funeral liturgy and it survives in the new Liturgy of the Hours for All Souls Day.
What is little known though is that in the Dominican Rite and in the Roman Rite prior to the Council of Trent, there was a Christmas Sequence known as Laetabundus. In fact, the liturgical "reformers' in the Concililum created by Pope Pius V after Trent did away, not only with many Sequences in the liturgy, but also with the "troped" Kyrie's which made its return in a small way in the Ordinary Form in one of the alternate penitential rites..."LORD, you were sent to heal the contrite of heart, LORD, have mercy."
If the Extraordinary Form could have any revision, it should not, in my opinion bring it closer to the revised but to correct some of what I believe to have been hasty revisions not only at Trent with the Sequences and tropes but more particularly in that of the Missal of 1962. Without even discussing the unfortunate Holy Week changes of 1955, other than the times of day for celebration, the 1962 Missal removed the Octave of the Epiphany which is at least one which should be restored and in the OF as well, with the Octave of Pentecost unfortunately abandoned in 1970. Bugnini loves his scissors. I tend to agree with Father Faber that "all change is bad from its very nature. It is full of evil; it unsettles and disturbs; it is full of the world; it is the very spirit of the world; and nothing worse can be said of it."
While the Sequence cannot be sung in the Mass, it can be done within it as a "hymn" or prior. If you attend the Toronto Oratory Church of the Holy Family at Midnight, I would expect it to be sung as in the past by the choir directed by Philip Fournier. Of course, you can always attend Christmas in Kinkora and hear it there too.
Courtesy of Music Sacra is the text newly set in the Gregorian. I've included below the English translation and the video of the Laetabundus extremely well sung and with perfection by Cantori Gregoriani of Italy. Enjoy.