The liturgical days follow the formula of what is call the concept of "Progressive Solemnity, " a continuum of importance from the greatest (solemnity) to the least (ferial).
The U.S. Bishops, in their recent document on music (Sing to the Lord, paragraphs 110-114), describe progressive solemnity this way:
Progressive solemnity means that‚ between the solemn, fuller form of liturgical celebration, in which everything that demands singing is in fact sung, and the simplest form, in which singing is not used, there can be various degrees according to the greater or lesser place allotted to singing.
Progressive solemnity includes not only the nature and style of the music, but how many and which parts of the rite are to be sung. For example, greater feasts such as Easter Sunday or Pentecost might suggest a chanted Gospel, but a recited Gospel might be more appropriate for Ordinary Time. Musical selections and the use of additional instruments reflect the season of the liturgical year or feast that is being celebrated. Solemnities and feasts invite more solemnity. Certain musical selections are more capable of expressing this solemnity, adding an extraordinary richness to these special celebrations. Such solemnity should never be allowed to devolve to an empty display of ceremony, however. The most solemn musical expressions retain their primary responsibility of engaging human hearts in the mystery of Christ that is being celebrated on a particular occasion by the Church. At other times, the liturgical season calls for a certain musical restraint. In Advent, for example, musical instruments should be used with moderation and should not anticipate the full joy of the Nativity of the Lord. In Lent, musical instruments should be used only to support the singing of the gathered assembly.
Click on underlined texts to find out more information about those words.
Sing to the Lord - A liturgical document by the U.S Bishops issued Nov. 14, 2007 which details the appropriate use of music in divine worship. This document supersedes any previous liturgical documents on music in Mass. Click here to download the entire document for your own personal reading.
Solemnities - This represents the highest of celebrations in the Church. The two of most importance being Easter and Christmas. For more information about solemnity go to www.newadvent.com.
Feasts - These celebrations commemorate important events in the life of Christ or important persons connected to Christ such as Mary or the Apostles. For more information about feasts go to www.newadvent.com.
Memorials - These are special celebrations that are worthy of rememberance by the Church. They commonly mark the anniversary of death of a saint. There are two kinds of memorials: obligatory and optional.
Obligatory Memorial - The Universal Church sees these days to be important enough to require their celebration. They are not to be displaced by other events or celebrations of secular purposes. They have particlar lectionary readings associated with them.
Optional Memorial - These memorials are part of the universal liturgical calendar, but their celebrations are not necessary. They are the lowest in the progression of solemnities. In fact, priests may choose to celebrate other memorials/events that are relevant to him, the local parish, the region, or diocese. See Wikipedia for more information.
Ferial Days - These days get their name from the Latin word, feria which mean, "free day." These days have no particular solemnity, feast, or memorial attached to them. They may be considered ordinary days. During these days the lectionary follows a continual progression through the Scriptures. Within these kinds of days are ranks. first, second, and third class ferial days. Ash Wednesday is a first class ferial day which gives way to no other Mass on that day.
Holy Days of Obligation - These are special solemnities that rank in such importance to the life of the community that the Church has deemed it necessary that all Catholics attend Mass on this day. See Wikipedia and for more information.
Liturgical Year - The calendar by which many Christians mark the dates of special celebrations in the year. See Wikipedia for more information.
To understand more fully the theology of liturgy, the Constitution on the Sacred Liturgy, Sacrosanctum Concilium.