A solemnity is the highest rank of celebration and is identified in the calendar with an S. Easter, Christmas, All Saints Day, the Ascension, Corpus Christi, and other celebrations of events in our Lord’s life on earth and certain saints are solemnities. The Gloria and Creed are always said on such days. Solemnities are like Sundays, though most of them are not days of obligation.
Feasts are the next rank down and are identified with an F. They consist of the celebration of certain saints like the feast of the Archangels or most of the Apostles. The Gloria is required on these days, but not the Creed.
Memorials are the lowest rank and are simply the celebrations of most of the saints. The Gloria is not said on these days. Obligatory memorials, identified with a capital M, must be observed. Optional memorials, identified with an m, may be observed but such observation is not required.