How he received an Embassy from the Persians and kept the Night Vigil with others at the Feast of Easter
In the meanwhile the Persians, hearing of the emperor's warlike preparations, and not a little terrified at the prospect of an engagement with his forces, dispatched an embassy to pray for conditions of peace. These overtures the emperor, himself a sincere lover of peace, at once accepted, and readily entered on friendly relations with that people. At this time, the great festival of Easter was at hand; on which occasion he rendered the tribute of his prayers to God, and passed the night in watching with the rest.
Source: Life of Constantine (New Advent)