The Cathedral of the Archangel Gabriel is celebrated by the Orthodox Church twice a year - the day after the Annunciation (April 8) and July 26.The Archangel Gabriel was chosen by the Lord in order to proclaim to the Virgin Mary, and with Her and to all people, the great joy of the Incarnation of the Son of God. Therefore, the next day after the Annunciation, having glorified the Most Pure Virgin, they thank the Lord and honor His messenger Archangel Gabriel. And the reason for the establishment of the second day of celebration (July 26), as it is believed, was the date of consecration in the 17th century in Constantinople of the temple, erected in the name of the holy Archangel.
The name Gabriel means "Power of God" or "God's stronghold." On the icons, the Archangel Gabriel is depicted with a branch of paradise, brought by him to the Virgin Mary, or with a luminous lantern in his right and a mirror in his left hand.
These days, all the appearances of the Archangel Gabriel are remembered: he inspired Moses when writing the book of Genesis; prophet Daniel announced the fate of the Jewish people; appeared to the righteous Anna with the news of the birth of the Virgin Mary from her; to the priest Zechariah he announced the birth of the Forerunner of the Lord; relentlessly abode with the Theotokos during Her earthly life; appeared to Saint Joseph the Betrothed.
In the Garden of Gethsemane, where Jesus Christ prayed, the Archangel appeared and strengthened Him before suffering. The myrrh-bearing women were the first to hear the joyful news of the Resurrection of Christ also from the Archangel Gabriel.
Source: https://www.calend.ru/holidays/0/0/1422/