The Solar Courts refers to the Dawn Court, the Day Court, and the Night Court because as opposed to being named for a season as the Spring, Summer, Autumn, and Winter Courts, it refers to a time of day.
While the Seasonal Courts experience only one season the Solar Courts experience all four seasons. And unlike what most expected, the Solar Courts do not alter the sun's path. Instead, the part of the day upon which the Court has been named is said to be the most beautiful. Thus, in the Dawn Court, the dawn would be the most beautiful, likewise for the Day Court and the Night Court.