Paschal troparion

The Paschal troparion or Christos anesti (Greek: Χριστός ἀνέστη) is the characteristic troparion for the celebration of the Orthodox Pascha (Easter) in churches that follow the Byzantine Rite. 
Like most other troparia, it is a brief stanza often used as a refrain between the verses of a psalm, but is also used on its own. It is sung in the first plagal (or fifth) tone. Its author or date is unknown, but it is assumed to be older than the 6th century.
Text.
The first line paraphrases from 1 Corinthians 15:20 ( Νυνὶ δὲ Χριστὸς ἐγήγερται ἐκ νεκρῶν).
The troparion is part of the Paschal Divine Liturgy of the Byzantine Rite, and it was certainly in use in the 5th or 6th century. 
Its ultimate origin is unknown; Metropolitan Hilarion (Alfeyev) (2009) has suggested a 2nd-century origin.
Usage.
According to the testimony of the Jerusalem tropologion (or "iadgari", an ancient hymnography surviving only in a Georgian translation of the 8th century), the troparion was sung at the end of the Easter Vigil in the late ancient Jerusalem Easter liturgy.
Based on the "Typikon of the Great Church", the troparion was part of the nonmonastic liturgy at the Hagia Sophia by the 10th century.
The troparion is first sung at the beginning of Matins on Pascha, following the procession around the church which precedes Matins. When all are gathered before the church's closed front door, the clergy and faithful take turns chanting the troparion, and then it is used as a refrain to a selection of verses from Psalms 67 and 117 (this is the Septuagint numbering; the KJV numbering is 68 and 118):
Let God arise, let His enemies be scattered; let those who hate Him flee from before His face (Ps. 68:1)<br>
As smoke vanishes, so let them vanish; as wax melts before the fire (Ps. 68:2a)<br> 
So the sinners will perish before the face of God; but let the righteous be glad (Ps. 68:2b)<br>
This is the day which the Lord hath made, let us rejoice and be glad in it. (Ps. 118:24)
In the remainder of the Vigil it is sung after each ode of the Paschal canon attributed to John Damascene, at the end of the Paschal stichera at the Aposticha, thrice at the dismissal of Matins, and at the beginning and end of the Paschal Hours. It is chanted again with the same selection of Psalm verses at the beginning of the Divine Liturgy, at the Little Entrance, during and following Communion, and at the dismissal of the Liturgy. It is then chanted again with the refrains at the beginning of Vespers, and at the dismissal of Vespers. This same pattern persists throughout Bright Week. 
Beginning on Thomas Sunday (the Sunday after Pascha), it is either sung or read thrice at the beginning of most services and private prayers in place of the usual invocation of the Holy Spirit, "O Heavenly King", and at the dismissals, during the 39-day paschal afterfeast; that is, up to and including the day before the Ascension of the Lord.
In recent years, the custom has developed of chanting the Paschal troparion in a number of different languages. 
In multicultural Orthodox parishes and communities, during the Paschal Vigil and during Vespers the following afternoon, it is not uncommon for the troparion to be performed in as many languages as are represented in the parish or as the choir can manage. 
In Finland, the Orthodox Church of Finland is a minority church. However, the Orthodox Easter night has, for many decades, been broadcast annually on radio and television, and thus the troparion has gradually become well-known to non-Orthodox Finns. In 1986, the Evangelical Lutheran Church of Finland—the largest religious denomination in the country—included the troparion in its revised official hymn book, where it is hymn number 90, as an Easter hymn. It is recommended to be sung three times in succession.