Christʼs saving work is celebrated in sacred memory by the Catholic Church on fixed
days throughout the course of the year. Through the yearly cycle the Church unfolds the entire mystery of Christ and keeps the anniversaries of the saints. Each day is made holy through the liturgical celebrations of the people of God.
As the events which are celebrated are multiple, with different degrees of importance, a ranking of celebrations has been developed:
Sunday: it's the most important because on Sunday, the Lord's Day, the Church celebrates the paschal mystery, having its origin from the day of Christʼs resurrection. Because of its special importance, the Sunday celebration gives way only to solemnities or feasts of the Lord.
Solemnities: these are the principal celebration days after Sundays. Easter and Christmas are the most known.
Feasts: a third degree. For example, saint John on the 27th of December.
Memorials: they can be either obligatory or optional. On Saturdays in Ordinary Time when there is no obligatory memorial, an optional memorial of the Blessed Virgin Mary is allowed.
Excerpt from the General Norms for the Liturgical Year and the Calendar, set by Pope Paul VI.
Note: in its article no. 59 you may find a most useful table of liturgical days.