Normally, Mass must be celebrated in a consecrated or blessed Church (private oratories or even rooms are allowed for special reasons: see Le Vavasseur, I, 200-4) and at a consecrated altar (or at least on a consecrated altar-stone). Mass may be celebrated on any day in the year except Good Friday at any time between dawn and midday. There are restrictions against private celebrations on Holy Saturday and in the case of private oratories for certain great feasts.  

A priest may say only one Mass each day, except that on Christmas Day when he may say three, and the first said immediately after midnight. In some countries (Spain and Portugal) a priest may also celebrate three times on All Souls' Day (2 November). Bishops may give leave to a priest to celebrate twice on Sundays and feasts of obligation. In cathedral and collegiate churches, as well as in those of religious orders who are bound to say the Canonical Hours every day publicly, there is a daily Mass corresponding to the Office and forming with it the complete cycle of the public worship of God. 

Two Main Types
A High (or sung) Mass requires a priest, deacon and choir. Low Mass, much more common, is the same service said by one priest.

Normally at Low Mass a server or acolyte helps the celebrant. Most of the text is invariable or ordinary, but certain parts, called proper, change with the occasion or day.

• Introit (Entrance Hymn)
• Anthem after the epistle (The Alleluia, gradual, tract, or sequence)
• The Offertory
• The Communion Hymns

• These are the Kyrie Eleison
• Gloria In Excelsis
• Credo
• Sanctus
• Agnus Dei.

These five ordinary hymns became a major musical form, called a Mass between from 1400 to 1700. The musical form ended with shift of interest to instrumental music. However, later composers did use the form.

Among the many composers who produced Masses are Beethoven, Palestrina, Mozart, Obrecht, Josquin, Monteverdi, Bach, Haydn, Verdi, and yes, even the irreverent Stravinsky.

Kyrie Eleison in Greek means, "Lord, have mercy." This is the first invariable hymn, and in traditional liturgy, the only one said in Greek. It has nine lines: Lord have mercy (thrice), Christ have mercy (thrice), Lord have mercy (thrice).

Gloria in Excelsis in Latin means, "Glory in the Highest." This Angelic Hymn or greater doxology is an ancient Christian hymn of thanks, praise and triumph for redemption. According to the Authorized Version, it begins with "Glory be to God on high, and on earth, peace, goodwill towards men," which is an amplification of Luke 2.14.

Creed in Latin means "I believe", is the declaration the belief of the basic doctrines of faith.

The Nicene Creed is usually described as a revision by the First Council of Constantinople (381) of the creed adopted at Nicaea in 325.

The Apostles' Creed has two material differences from the Nicene Creed: the phrase, "He descended into hell," is omitted in the Nicene, and the words "resurrection of the body" are changed to "resurrection of the dead" in the Nicene.

Sanctus in Latin means the "holy" hymn of the Roman Catholic Mass. The hymn's beginning "Holy, holy, holy," is from Isa. 6.3; Mat. 21.9. It is the solemn choral ending of the preface. The Sanctus also includes the Hosanna.

Agnus Dei in Latin means the "Lamb of God." Isaiah calls the expected Messiah the "Lamb of God." Jesus is met by John the Baptist with the words, "Behold, the Lamb of God, who takes away the sins of the world."

"At the consecration of a church at least one fixed altar must be consecrated. Altars, permanent structures of stone, may be consecrated at other times, but only in churches that have been consecrated or at least solemnly blessed. We have instances in which a simply priest has performed this rite. Walafridus Strabo, in the Life of St. Gall (ch. vi), says that St. Columban, at that time being a priest, having dedicated the church of St. Aurelia at Bregenz on the Lake of Constance, anointed the altar, deposited the relics of St. Aurelia under it, and celebrated Mass on it. But according to the present discipline of the Church, the ordinary minister of its consecration is the diocesan bishop. Without the permission of the ordinary, a bishop of another diocese cannot licitly consecrate an altar, although without such permission the consecration would be valid. One and the same bishop must perform the rite from the beginning to the end. An altar may be consecrated on any day of the year, but a Sunday or feast day is to be preferred (Pontificale Romanum). It is difficult to determine when the rite used at present was introduced. To the essentials of consecration reference is made as early as the sixth century by the Council of Agde (506): "Altars are to be consecrated not only by the chrism, but with the sacerdotal blessing"; and by St. Caesarius of Arles (d. about 542) in a sermon delivered at the consecration of an altar: "We have today consecrated an altar, the stone of which was blessed or anointed" (Migne, P.L., LXVII, Serm. ccxxx)."

1. Amice – A rectangular piece of linen with two long strips attached to two of its corners. This is worn if the alb does not cover the street clothes at the neck.
2. Alb – A long-sleeved white linen robe that reaches the ankles. Worn over the amice
3. Stole – A long strip of cloth worn over the neck as a symbol of office. Maybe white or the liturgical color of the day.
4. Cinture – A cord used to gather the alb at the waist.
5. Chasuble – An oval of highly decorated fabric with a hole for the head. This is the outermost vestment of the celebrating priest at the mass and reflects the color of the day or season.

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