The Roman Calendar was a lot different before the massive overhaul done by Pope S.D. Paul VI after Vatican II.
There were three fundamental rankings for calendar days: Duplex (double), Semiduplex (semi-double), and Simplex (simple). The practical difference between the three was abolished by Pope Bl. John XXIII, but since the Middle Ages, these rankings signified how the antiphons in the Office were prayed and how many nocturnes Matins had. On Duplex days, the antiphon is recited in full both before and after the psalm/canticle. On Semiduplex and Simplex days, half the antiphon (i.e. the part before the asterisk) is recited before the psalm/canticle, and then it is recited in full afterwards. The Duplex and Semiduplex both have three nocturnes at Matins, which means they have 9 lessons. The Simplex day has two nocturnes, so it has 3 lessons. Duplex and Semiduplex days also have I and II Vespers, whereas Simplex days have only I Vespers:
Double (antiphons doubled, 12 readings, two Vespers)
Semidouble (antiphons semidoubled, 12 readings, one Vespers)
Simple (antiphons semidoubled, 3 readings, one Vespers)
After the Council of Trent, Pope St. Pius V promulgated a new Roman Breviary which split Duplex days into four classes: I Class, II Class, and Double per Annum. Also, the Matins lessons were reduced to 9 for Doubles and Semidoubles. A little bit latter the Double per Annum was itself split into Major Double and Minor Double. The only functional difference between these classifications is which feast takes precedence if they overlap, although with St. Pius X’s reforms the lower-ranking feast can be commemorated in Divine Office with extra prayers. So, to summarize, in descending order:
Double
I Class
II Class
Major
Minor
Semidouble
Simple
Ferial
Seasonal weekday
Ordinary Time weekday (i.e. after Epiphany and Pentecost)
Pope Pius XII changed the terminology, so that Semidoubles became Simples and the Simples became Commemorations.
Pope John’s reforms in 1960 made it so the antiphons are always doubled, and all Sundays, and all feasts of the Third class (the former Major Doubles, Doubles and Semidoubles) are reduced to three readings at Matins. He also combined Major and Minor Doubles, Semidoubles, and Simple feasts into just “Double III Class”, and Commemorations are now IV Class:
Double
I Class
II Class
III Class
Commemoration
Paul VI just changed the terminology of it again; I Class became Solemnity, II and III Class became Feast, and Commemoration became Memorial.
