The whole elaborate ceremony of the Mass, which is such an enigma to the uninstructed, is nowhere vain or repetitious. Every word has its fitting relation to the whole; every gesture and genuflection, every change of vestments, has its symbolic significance. All the elements of the rite are merged into a unity under the sway of this central act of consecration and oblation. All the lessons, prayers, responses, and hymns are designed to lead up to it, to prepare the officers and people to share in it, and to impress upon them its meaning and effect. The architectural, sculptural, and decorative beauty of altar, chancel, and apse finds its justification as a worthy setting for the august ceremony, and as a fitting shrine to harbor the very presence of the Lord. The display of lights and vestments, the spicy clouds of incense, the solemnity of priestly chant, and the pomp of choral music, are contrived solely to enhance the impression of the rite, and to compel the mind into a becoming mood of adoration. There are several kinds of Masses, differing in certain details, or in manner of performance, or in respect to the occasions to which they are appropriated, such as the High Mass, Solemn High Mass, Low Mass, Requiem Mass or Mass for the Dead, Mass of the Presanctified, Nuptial Mass, Votive Mass, etc. The widest departure from the ordinary Mass form is in the Requiem Mass, where the Gloria and Credo are omitted, and their places supplied by the mediaeval judgment hymn, Dies Irae, together with certain special prayers for departed souls. In respect to the customary service on Sundays, festal, and ferial days there is no difference in the words of the High Mass, Solemn High Mass, and Low Mass, but only in the manner of performance and the degree of embellishment. The Low Mass is said in a low tone of voice and in the manner of ordinary speech, the usual marks of solemnity being dispensed with; there is no chanting and no choir music. The High Mass is given in musical tones throughout by celebrant and choir. The Solemn High Mass is performed with still greater ritualistic display, and with deacon, sub-deacon, and a full corps of inferior ministers. The prayers, portions of Scripture, hymns, and responses which compose the Catholic liturgy consist both of parts that are unalterably the same and of parts that change each day of the year. Those portions that are invariable constitute what is known as the Ordinary of the Mass. The changeable or “proper” parts include the Introits, Collects, Epistles and Lessons, Graduals, Tracts, Gospels, Offertories, Secrets, Prefaces, Communions, and Post-Communions. Every day of the year has its special and distinctive form, according as it commemorates some event in the life of our Lord or is devoted to the memory of some saint, martyr, or confessor.[52] Mass may be celebrated on any day of the year except Good Friday, the great mourning day of the Church.