'...Dies irae... the week before Advent.'
IIRC, the General Instruction of the Lectionary says "all other sequences are optional". Looked legalistically this is odd, since there are only four "official" sequences remaining: Victimae Paschale (obligatory), Veni Sancte Spiritus (obligatory), Lauda Sion (optional), and Stabat Mater (optional). If they meant "the Seqences for Corpus Christi and Our Lady of Sorrows are optional", they probably would have said that rather than "all other sequences".
If this is the sentence we're talking about: how exactly should GIRM have reworded it to exclude, for example, Petre summe Christi pastor?64. The Sequence which, except on Easter Sunday and on Pentecost Day, is optional, is sung before the Alleluia.
NB: this applies only for the OF; the EF only permits the five sequences of Easter, Pentecost, Corpus Christi, Our Lady of Sorrows, and Requiems.
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