With that said, I thought it might be of some interest to review some liturgical notes related to All Souls specifically and also the liturgy for the dead generally. We begin with Archdale King's account:
In the first instance, Masses for the dead were indistinguishable from other Masses, and it was only later that they became specifically 'requiems'. Amalarius (ob. 853) speaks of the observance of a day in commemoration of the faithful departed (De. eccles. Offic. lib IV, cap. XLII), but it was left to Odilio, abbot of Cluny (ob. 1049), to fix 2 November for all the houses of his Order. The day was first observed in 998, and was approved by Popes Sylvester II (999-1003), John XVIII (1004-9) and Leo IX (1048-54). The commemoration had been accepted everywhere by the 13th century, as Durandus (ob. 1296) shows. Priests of the kingdom of Aragon were permitted by Paul III (1534-49) or Julius III (1550-5) to say two Masses on this day, and regular clergy might say three. The constitution Quod expensis (1748) of Benedict XIV (1740-58) extended the privilege of three Masses for all priests in Spain and Portugal, and Leo XIII (1878-1903) in Trans oceanum included those of Latin America. In 1915, Benedict XV (1914-22) in the constitution Momentum authorised priests throughout the world to triplicate on 2 November.
-- Archdale King, Liturgy of the Roman Church, p. 206-7
On the matter of the Dies irae sequence, King comments:
Dies irae, a magnificent poem on the Day of Judgement, has been ascribed to Thomas of Celano (ob. c. 1250), one of the early followers of St. Francis of Assisi. Like the Stabat Mater, it was originally intended to serve as a private devotion, but it is already found in some 13th century missals as a sequence in Masses for the dead. In the following century the Dies irae was sometimes provided as a sequence for the first Sunday of Advent, where it served as a fitting prelude to the gospel of the Second Coming.
-- Ibid., p. 256
Blessed Ildefonso Schuster likewise offers the following on the subject of the liturgy at the graveside in Christian antiquity, which, while not specifically about All Souls, is nonetheless of related interest:
From the earliest times, our Mother the Church accompanied her children to the grave with liturgical rites, which soon became traditional... the liturgy did not destroy but elevated and sanctified the funeral traditions and classical customs of the Greeks and Romans, who were used to surround their dead with such poetical demonstrations of piety and affection.
The deceased having been buried outside the city, but not so far as to prevent his friends from frequently visiting the grave, the burial service was followed in ancient times by nine days of mourning known, therefore, as novemdialia. The third and ninth of these days were the most solemn, for then the relatives celebrated the funeral banquet and sacrifice at the tomb. But throughout the year, the parentalia -- a kind of annual commemoration of all the dead -- the rosalia, the dies violationis, the birthday, etc., were all occasions when the relations assembled round the tomb of their dear one, and scattered flowers, perfumes and aromatic herbs upon it, mingled with their tears.
It is signifcant that whilst the pagans celebrated as the chief anniversary the dies natalis, or birthday of the dead, the Christians always meant by the dies natalis the day of death, or rather the birth of the faithful to eternal life.
According to the Apostolic Constitutions, the third, ninth and fortieth day after the decease were celebrated with liturgical rites. St. Ambrose mentions this tradition, but the great Doctor of Milan knew, too, that another existed by which the third and the thirtieth day instead of the seventh and fortieth were observed...
Although these frequent pilgrimages to the grave... mentioned by St. Jerome on the occasion of St. Paul the Hermit, the funeral Agape, the libations of perfumes, the rosalia and violationes were a suggestive part of early Christian observances, yet the central rite of the liturgy for the dead was the eucharistic Sacrifice.
-- Ildefonso Schuster, The Sacramentary, pp. 231-2
These latter observances, incidentally, are intimated by Prudentius in the last stanza of his Hymn for the Burial of the Dead (Hymnus ad Exequias Defuncti):
But we will honour our dear dead
With violets and garlands strown,
And o'er the cold and graven stone
Shall fragrant odours still be shed.
