The Synaxis of the Chief of the Heavenly Hosts, Archangel Michael and the Other Heavenly Bodiless Powers: Archangels Gabriel, Raphael, Uriel, Selaphiel, Jehudiel, Barachiel, and Jeremiel was established at the beginning of the fourth century at the Council of Laodicea, which met several years before the First Ecumenical Council. 

http://oca.org/saints/lives/2015/11/08/103244-synaxis-of-the-archangel-michael-and-the-other-bodiless-powers