Council of Nicaea

June 12–The Ecumenical Council of Nicaea, A.D. 325

From the LCMS website:

The first Council of Nicaea was convened in the early summer of 325 by the Roman Emperor Constantine at what is today Isnuk, Turkey. The emperor presided at the opening of the council. The council ruled against the Arians, who taught that Jesus was not the eternal Son of God but was created by the Father and was called Son of God because of his righteousness. The chief opponents of the Arians were Alexander, bishop of Alexandria, and his deacon, Athanasius. The council confessed the eternal divinity of Jesus and adopted the earliest version of the Nicene Creed, which in its entirety was adopted at the Council of Constantinople in 381.

May 21–Emperor Constantine, Christian Ruler, and Helena, Mother of Constantine

From the LCMS website:

Constantine I served as Roman Emperor from A.D. 306 to 337. During his reign the persecution of Christians was forbidden by the Edict of Milan in 312, and ultimately the faith gained full imperial support. Constantine took an active interest in the life and teachings of the church and called the Council of Nicaea in 325 at which orthodox Christianity was defined and defended. His mother, Helena (ca. 255-329), strongly influenced Constantine. Her great interest in locating the holy sites of the Christian faith led her to become one of the first Christian pilgrims to the Holy Land. Her research led to the identification of Biblical locations in Jerusalem, Bethlehem, and beyond, which are still maintained as places of worship today.