According to the Synaxarion of Constantinople, the following commemoration is made on August 8th:
Consecration of the Church of the Apostles Peter and Paul, within the walls of the Holy Apostle and Evangelist John the Theologian, inside the revered house of the Most Holy Theotokos in Marnakiou. And the Venerable Cassian. And we commemorate when the eclipse of the sun took place from the sixth to the ninth hour, so that even the stars appeared, in the year from the creation of the world six-thousand three-hundred and ninety-nine [6399, or the year 891 A.D.], at the fifteenth solar cycle, as well as lunar cycle, at the ninth Indiction, in the reign of Leo the Wise, and Alexander the brother of this pious and friend of Christ emperor.
This is the only day in the Church calendar that an eclipse is commemorated, let alone a total solar eclipse that lasted three hours (probably from the beginning of the passing to the end, notably from noon to 3:00pm). It seems to have taken place on the day of the consecration of the Church of the Apostles Peter and Paul in Marnakiou, which is near Chalcedon. Though many significant eclipses are recorded in Constantinople's history, including one which took place a week before the Fall of Constantinople, this is the only one commemorated in the Church calendar, probably because it lasted three hours and took place during the consecration. That this was a total solar eclipse is suggested by the fact that stars were visible in the middle of the afternoon.
It should be noted that the hymns at Vespers today speak of the sun appearing dim compared to the divine light of the Transfiguration. In His compassion for humanity, Christ took on mortal flesh, yet Peter, James and John saw the radiance of His glory. The incarnate Lord “made Adam’s darkened image to shine again” when He appeared on earth “arrayed in the original beauty of the Image" (Genesis 1:26). August 8th falls within the post-festal days of the Transfiguration.