Alleluia in the context of "Christ the Lord Is Risen Today"

In medieval music, ambitus (Ecclesiastical Latin: ['am.bi.t̪us]) is a Latin term literally meaning enclos[ur]e, and in Medieval Latin means the "range" of a melodic line, most usually referring to the range of scale degrees attributed to a given mode, particularly in Gregorian chant. In Gregorian chant specifically, the ambitus is the range, or the distance between the highest and lowest note. Different chants vary widely in their ambitus. Even relatively florid chants like Alleluias may have a narrow ambitus. Earlier writers termed the modal ambitus "perfect" when it was a ninth or tenth (that is, an octave plus one or two notes, either at the top or bottom or both), but from the late fifteenth century onward "perfect ambitus" usually meant one octave, and the ambitus was called "imperfect" when it was less, and "pluperfect" when it was more than an octave.

All of the church modes are distinguished in part by their ambitus. The plagal modes have the final in the middle of the ambitus, while the authentic modes generally go no more than one note below the final.

The ordinary, in Catholic and Lutheran liturgy, refers to the part of the Mass or of the canonical hours that is reasonably constant without regard to the date on which the service is performed. It is contrasted with the proper, which is that part of these liturgies that varies according to the date, either representing an observance within the liturgical year, or of a particular saint or significant event, or to the common which contains those parts common to an entire category of saints such as apostles or martyrs.

The ordinary of both the Mass and the canonical hours does, however, admit minor variations following the seasons (such as the omission of "Alleluia" in Lent and its multiple additions in Eastertide). These two are the only liturgical celebrations in which a distinction is made between an ordinary and other parts. It is not made in the liturgy of the other sacraments or of blessings and other rites.

The Easter Vigil, also known as the Paschal Vigil, the Great Vigil of Easter, or Holy Saturday at the Easter Vigil on the Holy Night of Easter, is a liturgy held in traditional Christian churches as the first official celebration of the Resurrection of Jesus. Historically, it is during this liturgy that people are baptized and that adult catechumens are received into full communion with the Church. It is held in the hours of darkness between sunset on Holy Saturday and sunrise on Easter Day – most commonly in the evening of Holy Saturday or midnight – and is the first celebration of Easter, days traditionally being considered to begin at sunset.

Among liturgical Western Christian churches including the Roman Catholic Church, the Lutheran Churches and the Anglican Communion, the Easter Vigil is the most important liturgy of public worship and Mass of the liturgical year, marked by the first use since the beginning of Lent of the exclamatory "Alleluia", a distinctive feature of the Easter season.