The Trisagion Service is a brief memorial service celebrated by a priest on behalf of deceased Orthodox Christians.
Trisagion (Greek: τρισάγιον) means “Thrice Holy” and is the hymn of the angels, “Holy! Holy! Holy!” (Isaiah 6:1).
The service is called this because it begins with the familiar prayer, “Holy God! Holy Mighty! Holy Immortal! Have mercy on us,” said three times.
After the beginning prayers, a series of hymns, a litany, and a petition are chanted, asking the merciful Lord to give rest to the reposed among the Saints. The service concludes with the dismissal blessing and singing, “May your memory be eternal.” The family may bring Kollyva to be blessed and distributed among the people.
The service is typically celebrated on the night before the funeral. It can be offered anytime, usually immediately after the person has died, after the funeral, the 3rd, 9th, & 40th days, the 3rd, 6th, 9th, & 12th months, and annually with the larger Memorial Service called a “Mnemosyna” (Greek: Μνημόσυνα) — which means “calling to mind” because they are prayers for the departed.
Saint John Chrysostom says, “The inhabitants of heaven and earth are brought together in a common assembly; there is one thanksgiving, one shout of delight, one joyful chorus.” In praying the Trisagion, we unite heaven and earth, angels and mankind, Saints and spiritual strugglers, the living and the deceased.
Source: GOARCH