The Synaxis of the Archangel Gabriel is celebrated on the day after the Annunciation, and a second time on July 13. It was instituted in the ninth century, perhaps to celebrate the dedication of a church at Constantinople. Originally, the Feast was observed on October 16 (Juan Mateos, Le Typikon De La Grande Eglise).
An account of the Holy Archangel Gabriel is found under March 26 and November 8.
It is believed that the Synaxis of the Archangel Gabriel was transferred to this day from March 26 so that it could be celebrated more festively than in the period of the Great Fast; and, in fact, all the miracles of the Archangel are celebrated on this feast day, which has been listed here in the church books since the ninth century.
Apolytikion of Synaxis of Archangel Gabriel
O Commanders of the Heavenly Host, we the unworthy beseech you, that through your entreaties you will fortify us, guarding us in the shelter of the wings of your ethereal glory, even as we fervently bow before you crying: “Deliver us from all danger, as Commanders of the Powers on high! “
Kontakion of Synaxis of Archangel Gabriel
Plagal of the Second Tone
As thou beholdest the glory of God in Heaven, and on earth dost bestow grace from on high, O leader of Angels, wise Gabriel, minister of the glory of God, and divine defender of the world, save and keep them that cry to thee: Be thyself our helper, and no one can be against us.
Source: oca.org / goarch.org / westserbdio.org