From "Liturgical Meditations for the Entire Year" by the Sisters of St. Dominic, Adrian, MI (B. Herder, 1960). Via Breviarium S.O.P:
The magnanimous spirit of our Order inspires devotion to the holy souls in purgatory. Love for the Church suffering, deeply rooted in the soul of St. Dominic, has been preserved for centuries in the traditions, Constitutions, and liturgy of the Order of Preachers. Today throughout our Order the Mass and Office of the Dead will be offered for the souls of Dominican priests, brothers, and sisters, who are now awaiting their release from the pains of purgatory.Image Source: Opeast.org
Our Dominican brothers and sisters are asking today for our prayers. The Office of the Dead is one contribution we can make to their needs, but it is very little compared with what we have within our means to give. Because it is the special suffrage assigned by our Constitutions, we owe it as a matter of justice. If we look forward to the careful performance of this duty today, we shall find it a joy to offer this and and many other acts of prayer and charity for the souls of our beloved departed. "O God, the giver of pardon and the author of human salvation, we beseech Thy clemency to admit the brothers and sisters of our congregation...to the fellowship of eternal bliss" (Office of the Dead).
The souls in purgatory are making reparation for the temporal punishment due to their sins. As some sins are more serious than others, the punishment for some is of longer duration. Likewise some sins are of greater adherence in the soul than others, according as man is attached to them and more inclined to commit them. Therefore the sins that adhere more strongly to the soul are purged more slowly (Summa Theologica, Supplement, Appendix II, a.8.).
From this teaching of St. Thomas we learn that we must never cease to pray for our departed brothers and sisters, because they may be detained for a long time in purgatory. Although they lived holy lives and served God faithfully, attachment to venial sins may be separating them from the beatific vision. Let us be generous in our prayers for them and honest in our examination of conscience lest attachment to sloth, criticism, and disobedience may become habitual in our lives and require a lengthy and painful purgatory."We offer to Thee, O Lord, sacrifice of praise and prayers; do Thou receive them in behalf of those souls whom we commemorate this day." (Offertory of the Mass for the Dead).