Typically saints are remembered on the anniversary of their death, as it recalls their entrance into Heaven. However, sometimes that date is on an already existing feast and the celebration is transferred to the closest available day.
In the case of St. Dominic, he died on August 6, which is the feast of the Transfiguration of the Lord.
This date has great significance, as St. Dominic is highly regarded as one of the most influential saints in the Western Church.
Furthermore, many recognized St. Dominic's extraordinary holiness during his life and even noted how he was a man "full of light."
Sr. Claire, OP, at the Queen of Peace Monastery recalls a description of St. Dominic written by Bl. Cecilia: "From his brow and eyes emanated a kind of radiance which drew everyone to revere and love him."
She also notes, "Dominic can be recognized, from among the saints, in the images and iconography as the one with a star shining near his head and often from his forehead."
This particular feature of religious art is a reference to the story of St. Dominic's baptism. According to Providence College, "when St. Dominic was a baby his godmother saw a star on his forehead during the baptism."
In many ways St. Dominic was "transfigured" by Jesus during his life and let the light of his life shine upon all he encountered.
It is then very fitting that St. Dominic died on the Transfiguration, as he was able to finally experience the fulness of Jesus' light on that day in the eternal joy of Jesus' presence.