That the Council at Nicæa was held in the Twentieth, the Dedication of the Church at Jerusalem in the Thirtieth, Year of Constantine's Reign
This second synod the emperor convened at Jerusalem, being the greatest of which we have any knowledge, next to the first which he had summoned at the famous Bithynian city. That indeed was a triumphal assembly, held in the twentieth year of his reign, an occasion of thanksgiving for victory over his enemies in the very city which bears the name of victory. The present meeting added luster to the thirtieth anniversary, during which the emperor dedicated the church at the sepulchre of our Saviour, as a peace-offering to God, the giver of all good.
Source: Life of Constantine (New Advent)