The "passion" services are so named for the fact that their content is taken from the services of Passion Week. These are regular lenten vespers of Sunday night with hymns taken from Passion Week and readings of the four Gospels describing the suffering of Christ. In some churches, an akafist to the Passion of Christ is sometimes also served.
God willing, we will have these services four times in the course of Great Lent, as each time, the focal point is the reading of one of the four Passion Gospels from each evangelist.
The service, like that of Forgiveness Sunday, begins as a festal service and then changes (vestments and melodies) to that of Great Lent. At the end, the Gospel is brought out into the center of the church, in front of the Crucifixion, and is read and the faithful venerate it.