Chapter II-a. Liturgical Days
I. The Liturgical Day in General
3. Each day is made holy through the liturgical celebrations of the people of God,
especially through the eucharistic sacrifice and the divine office.
The liturgical day runs from midnight to midnight, but the observance of Sunday
and solemnities begins with the evening of the preceding day.
4. The Church celebrates the paschal mystery on the first day of the week, known
as the Lord's Day or Sunday. This follows a tradition handed down from the apostles
and having its origin from the day of Christ's resurrection. Thus Sunday must be ranked
as the first holyday of all. 
5. Because of its special importance, the Sunday celebration gives way only to
solemnities or feasts of the Lord. The Sundays of the seasons of Advent, Lent, and Easter,
however, take precedence over all solemnities and feasts of the Lord. Solemnities occuring
on these Sundays are observed on the Saturdays preceding.
6. By its nature, Sunday excludes any other celebration's being permanently assigned
to that day, with these exceptions:
a. Sunday within the octave of Christmas is the feast of the Holy Family;
b. Sunday following 6 January is the feast of the Baptism of the Lord;
c. Sunday after Pentecost is the solemnity of the Holy Trinity;
d. the last Sunday in Ordinary Time is the solemnity of Christ the King.
7. In those places where the solemnities of Epiphany, Ascension, and Corpus Christi
are not observed as holydays of obligation, they are assigned to a Sunday, which is then
considered their proper day in calendar. Thus:
a. Epiphany, to the Sunday falling between 2 January and 8 January;
b. Ascension, to the Seventh Sunday of Easter;
c. the solemnity of Corpus Christi, to the Sunday after Trinity Sunday.
III. Solemnities, Feasts, and Memorials
8. As it celebrates the mystery of Christ in yearly cycle, the Church also venerates
with a particular love Mary, the Mother of God, and sets before the devotion of the
faithful the memory of the martyrs and other saints. 
9. The saints of universal significance have celebrations obligatory throughout the entire
Church. Other saints either are listed in the General Calendar for optional celebration or
are left to the veneration of some particular Church, region, or religious family. 
10. According to their importance, celebrations are distinguished from each other and
named as follows: solemnities, feasts, memorials.
11. Solemnities are counted as the principal days in the calendar and their observance
begins with evening prayer I of the preceding day. Some also have their own vigil Mass
for use when Mass is celebrated in the evening of the preceding day.
The celebration of Easter and Christmas, the two greatest solemnities, continues for
eight days, with each octave governed by its own rules.
13. Feasts are celebrated within the limits of the natural day and accordingly do not
have evening prayer I. Exceptions are feasts of the Lord that fall on a Sunday in Ordinary
Time and in the Christmas season and that replace the Sunday office.
14. Memorials are either obligatory or optional. Their observance is integrated into the
celebration of the occurring weekday in accord with the norms set forth in the General
Instructions of the Roman Missal and the Liturgy of the Hours.
Obligatory memorials occurring on Lenten weekdays may only be celebrated as optional
Should more than one optional memorial fall on the same day, only one may be celebrated;
the others are omitted.
15. On Saturdays in Ordinary Time when there is no obligatory memorial, an optional
memorial of the Blessed Virgin Mary is allowed.
16. The days following Sundays are called weekdays. They are celebrated in different ways
according to the importance each one has.
a. Ash Wednesday and the days of Holy Week, from Monday to Thursday inclusive, have precedence
over all other celebrations.
b. The weekdays of Advent from 17 December to 24 December inclusive and all the weekdays
of Lent have precedence over obligatory memorials.
c. All other weekdays give way to solemnities and feasts and are combined with memorials.