The Monastic Diurnal is the day hours of the traditional Benedictine Office: Lauds, Prime, Terce, Sext, None, Vespers, Compline. It excludes Vigils which would be in a separate book; very few people outside a monastery would pray the traditional Benedictine vigil of 12 psalms and two long readings (and 3 canticles, the gospel, Te Deum and another long reading or homily at Sundays and feasts). The traditional Monastic Diurnal, that is using a publication before Vatican-II, would not follow the Roman Calendar but the Monastic Calendar, which has saints and feasts proper to the Benedictines and would follow the pre-Vatican II liturgical year (i.e. includes Septuagesima, no OT but instead "weeks after Epiphany and “weeks after Pentecost”).
The Traditional Monastic Diurnal can only be prayed, licitly, in Latin as there is no approved vernacular translation. Any vernacular translation will be for the sake of understanding only, it has to be prayed in Latin.
However the Benedictine Office underwent renovation after Vatican II. It maintains the traditional psalm layout, but underwent the following changes:
- The option to omit Prime and distribute its psalms to the other minor hours or Vigils
- Update of the Liturgical Year to match the current practice (Advent, Christmas, Ordinary time, Lent, Eastertide, Ordinary time)
- Update of all collects, intercessions, etc, to match the current LOTH
- Addition of new saints to the calendar, removal of certain saints no longer on the calendar (similar to the Roman Office but still proper to the Benedictines)
- OT canticle at Lauds matching the current LOTH
- Option to place the hymn of Lauds, Vespers and Compline at the start of the Office (or retain it after the responsory for Lauds and Vespers, and after the psalmody at Compline)
- Option to include the responsory and Gospel canticle (Nunc dimittis) at Compline (traditionally the Monastic Office has neither, both are replaced by a simple verse and response).
- Where approved translations exist, the ability to use the vernacular.
The important thing is that the office continues to use the traditional Benedictine psalm schema laid out by St. Benedict in his 1500 y.o. rule: all psalms said in one week, certain psalms repeated every day (3 before the invitatory, 66, 50, 148-150 at Lauds, 4, 90 and 133 at compline, and the Gradual Psalms at the minor hours).
There are 3 other Benedictine schemas but they are completely post-Vatican II schemas. One of them, Schema B (used by our abbey) retains all 150 psalms in a week, which the Rule of St. Benedict allowed for.