The Day Court is one of the seven courts of Prythian, and one of the three solar courts. This court was one of the three courts, along with the Winter Court and the Summer Court, to rebel against Amarantha's control over Prythian. The High Lord of the Day Court, Helion Spell-Cleaver, is a dark-skinned man clothed in white and gold colors. He is also Lucien's biological father from an affair he had with the Lady of the Autumn Court hundreds of years before the start of A Court of Thorns and Roses.
In A Court of Mist and Fury, Rhysand tells Feyre that the Day Court had a thousand libraries that were personally looted by Amarantha, and that the Day Court has been responsible for archiving Prythian's knowledge of magic for centuries. The Day Court is known for its immense basin of knowledge and it's people for their gift of inventing.
The Day Court is bordered to the north by the Night Court, and to the south by the Dawn Court.
In A Court of Wings and Ruin, Helion gave a piece of his magic to Rhysand, so he could come back from the dead.