Replies 37-40 in this thread has the forms in the Armenian tradition:http://www.orthodoxchristianity.net/forum/index.php/topic,13200.msg359874.html#msg359874
It changes according to the time of year, what is being celebrated, etc. During the liturgy it is sung during the "Little Entrance" (I think that is what it is called,) when the Gospel book is being carried around the altar, before the Gospel is read. It is sung during other services also.
The Trisagion tag below will bring you to other threads about the prayer.