Today, tomorrow, and the day after give us some insight into how feasts have been celebrated and ranked throughout the history of the Roman Church. The Roman rite generally has four categories for days:
Semi-Double is a compromise between Simple and Double. A Ferial day uses the psalter of the day (including the 12 Mattins psalms), the full preces and suffrages during the major and minor hours (intercessory prayers), and celebrates the Mass proscribed for that day. For most of the year this is a repetition of Sunday's Mass. During Lent it is a unique Mass daily. Mass uses an odd number of orations, usually three, but enough commemorations can boost it to five or seven.
A Simple usually celebrates a lesser known saint of local interest and has much the same character as a Ferial day, except the psalms at Lauds are festive and the Mass is of the saint rather than of the previous Sunday. Mass is sung after Sext on Ferial and Simple days, except when the Ferial or Simple day has a penitential character or is a vigil—like next week's vigil for St. Lawrence—in which case the Mass is after None. During Lent the Ferial days enjoy privilege and supersede the Simple feasts. On Simples the full preces are sung, but the Commemoration of the Cross is omitted during the suffrages. A Simple begins with Vespers the previous night and ends at None in the afternoon, as is still the Eastern practice for feasts.
A Double begins with first Vespers the previous night and ends with Compline the night of the feast. Mattins uses 9 psalms and 9 lessons in three nocturnes as Mattins, using either psalms proper to the feast or from the Commons. Lauds are of course festive and there are no suffrages or preces during any of the hours of the Office. Mass, sung after Terce, has no votive orations, although it can admit commemorations of impeded feasts or Ferial days.
Semi-Double is, again, a blend of Double and Simple. It retains all the psalms and the arrangement or the hours of a Double, but with the suffrage, preces, and commemoration rules of a Simple. The Rad Trad always fancied Semi-Double to be an arrangement invented by monks who thought a saint was important, but not important enough to impede Sunday. Sundays are Semi-Double, but take priority over any Semi-Double saint's feast.
What does this have to do with the period from August 4-6th? Every day of that period is a Double feast. Today's feast of St. Dominic began last night, overtaking Sunday's Vespers. Tomorrow is the feast of Our Lady of the Snows (St. Mary Major in Rome). Finally, on Wednesday, is the Transfiguration of the Lord. Despite their similar celebration in the Mass and Office they are not observed as having the same status.
Many will have heard of the ranks "Double I Class," "Double II Class," "Great Double" etc. These days illustrate the significance of these ranks. They are means of classifying the prominence of feasts that are celebrated the same way in the Office. It seems that when these classifications were given to Double feasts in the mid-second millennium—complete when Clement VIII instituted "Greater Double" rank—those in charge of the process began with the understanding that the feast of the Transfiguration would be the most significant day in the sequence. From there they also understood that while not as important, Our Lady of Snows was both a Marian feast and a major feast within the city of Rome. St. Dominic was an important enough saint to impede the Sunday Mass and Office, as he would have until 1911, but not enough to impede the other feasts. So St. Dominic retained a regular Double rank, while Our Lady of the Snows became Greater Double, and the Transfiguration was kept as a Double II Class. Hence second Vespers of St. Dominic is replaced by first Vespers of Our Lady of the Snows, which in turn loses its second Vespers to the first Vespers of the Transfiguration (which enjoys first and second Vespers).
All this becomes apparent when one sees no Greater Double feast in the kalendar is surrounded by Ferial or Simple days!
A picture of St. Mary Major from the Rad Trad's personal collection.
From the Baptistry.