SOLEMNITIES and FEASTS
We as Catholics commonly use the word "feast" to cover everything from church feasts of various saints and the Blessed Mother, to Corpus Christi, etc. We also understand that there are three kinds of feasts/celebrations: memorial, feast, solemnity.
Effectively we use the word "feast" to cover all levels of celebration, even though the word also has a precise technical meaning in the hierarchy of celebrations. There is no great difficulty in this, as the context usually clarifies whether we are speaking technically or in general.
The difference between the three basic categories resides in their importance, which in turn is reflected in the presence or absence of different liturgical elements.
Solemnities are the highest degree and are usually reserved for the most important mysteries of faith. These include Easter, Pentecost and the Immaculate Conception; the principal titles of Our Lord, such as King and Sacred Heart; and celebrations that honor some saints of particular importance in salvation history, such as Sts. Peter and Paul, and St. John the Baptist on his day of birth.
Solemnities have the same basic elements as a Sunday: three readings, prayer of the faithful, the Creed and the Gloria which is recited even when the solemnity occurs during Advent or Lent. It also has proper prayer formulas exclusive to the day: entrance antiphon, opening prayer, prayer over the gifts, Communion antiphon, and prayer after Communion. In most cases it also has a particular preface.
Some solemnities are also holy days of obligation, but these vary from country to country. A solemnity is celebrated if it falls on a Sunday of ordinary time or Christmastide. But it is usually transferred to the following Monday if it falls on a Sunday of Advent, Lent or Easter, or during Holy Week or the Easter octave.
A feast honors a mystery or title of the Lord, of Our Lady, or of saints of particular importance (such as the apostles and Evangelists) and some of historical importance such as the deacon St. Lawrence.
The feast usually has some proper prayers but has only two readings plus the Gloria. Feasts of the Lord, such as the Transfiguration and Exaltation of the Holy Cross, unlike other feasts, are celebrated when they fall on a Sunday. On such occasions they have three readings, the Gloria and the Creed.
A memorial is usually of saints but may also celebrate some aspect of the Lord or of Mary. Examples include the optional memorial of the Holy Name of Jesus or the obligatory memorial of the Immaculate Heart of Mary.
From the point of view of the liturgical elements there is no difference between the optional and obligatory memorial. The memorial has at least a proper opening prayer and may have proper readings suitable for the saint being celebrated. The readings of the day may be used, and the lectionary recommends against an excessive use of specific readings for the saints so as not to interrupt too much the continuous cycle of daily readings.
On the other hand, the specific readings should always be used for certain saints, above all those specifically mentioned in the readings themselves, such as Martha, Mary Magdalene and Barnabas.
During Lent and Advent from Dec. 17 to 24 memorials may be celebrated only as commemorations. That is, only the opening prayer of the saint is used and all the rest comes from the day.
Nov. 2, All Souls' Day, is something of a special class that, without being a solemnity, still has precedence over a Sunday.
It is also important to note that the same celebration may have a different classification in various geographical areas, as some celebrations and saints are venerated more in one place than in another. For example, St. Benedict, an obligatory memorial in the universal calendar, is a feast in Europe since he is one of its patrons. But he rates a solemnity in the diocese and abbey of Montecassino where he is buried.
Finally, the decision on whether a solemnity such as the Body and Blood of the Lord is a holy day of obligation falls primarily upon the bishops' conference, which decides based on the pastoral reality of each country. Some have maintained the traditional Thursday celebration and kept it as a holy day; others might have maintained the day but without the obligation. Many have preferred to transfer the celebration to the following Sunday so as to ensure its celebration with the greatest number of faithful.
The Vatican, for example, continues the traditional Thursday celebration and thus the Holy Father's procession with the Blessed Sacrament is held on that day. The Diocese of Rome, however, along with the rest of Italy, celebrates it on the following Sunday.