A solemnity is the highest rank of celebration and is denoted by a S on the calendar. Easter, Christmas, All Saints Day, the Ascension, Corpus Christi, and other celebrations of events in our Lord's earthly life are solemnities. On these days, the Gloria and the Creed are always said. They are similar to Sundays, though solemnities are not always days of obligation.

The next tier down are feasts, denoted with an F. They include commemorating certain saints, such as the Archangels' feast or the apostles' majority. These days, the Gloria is necessary, but not the Creed.

The majority of the saints are celebrated as memorials, which have the lowest rank. These days, the Gloria is not proclaimed. Memorials denoted with a capital M must be observed. Memorials denoted with a m are optional.

From Catholic Answers