"On Forgiveness Sunday many attend Forgiveness Vespers on the eve of Great Lent. They hear on the Lord's teaching about fasting and forgiveness and enter the season of the fast forgiving one another so that God will forgive them. If you forgive men their trespasses, your heavenly Father will forgive you; but if you do not forgive men their trespasses, neither will your heavenly Father forgive you your trespasses (Matthew 6:14).
After the dismissal at Vespers, the priest stands beside the analogion, or before the ambon, and the faithful come up one by one and venerate the icon, after which each makes a prostration before the priest, saying, "Forgive me, a sinner." The priest also makes a prostration before each, saying, "God forgives. Forgive me." The person responds, "God forgives," and receives a blessing from the priest. Meanwhile the choir sings quietly the irmoi of the Paschal Canon, or else the Paschal Stichera. After receiving the priest's blessing, the faithful also ask forgiveness of each other." (Orthodox WIKI)
Forgive me a sinner.