Prior to the revision of the Breviary instituted by Pope Pius XII, the Octave of Our Holy Father St. Dominic would be celebrated from August 5 till August 11. This is one of the many octaves of the Dominican saints that were casualties of the liturgical revisions of the mid-20th centuries. Not all of the Dominican saints had "solemn" octaves after their feast day, but the major ones like St. Dominic, St. Catherine of Sienna, St. Thomas Aquinas, and St. Peter Martyr did.
On August 5, at Pretiosa, we read in the Martyrology for August 6 the commemoration of his death on the actual day that he passed to his eternal reward:
At Bologna, the birthday of St. Dominic, confessor and founder of the Order of Friars Preachers. He was a man of great renown for holiness and learning. Until death he preserved without stain his virginity, and by the singular grace of his merits raised three dead men to life. By his preaching he curbed heresies, and established many persons in a religious and godly manner of life. He died on this day, but his festival, by an ordinance of Pope Paul IV, is celebrated on August 4.