There are several antonyms for the phrase "crack dawn", which is often used to describe the early morning hours when the sun is just starting to appear on the horizon. One antonym for crack dawn could be "late night", which refers to the hours after midnight and into the early morning when many people are still awake. Another antonym could be "midday", which is the middle of the day when the sun is at its highest point in the sky. Finally, "sunset" could be an antonym for "crack dawn", as it refers to the time just before the sun sets below the horizon in the evening.