Union Day was a celebration that the Cardassian people participated in and involved them honoring the day of the formation of their society.

During the official first contact between the Bajoran and Cardassian races, Darrah Mace asked Skrain Dukat if the Cardassian's had any festivals they celebrated. The Dalin replied by giving them example of Union Day. (TLE - Terok Nor novel: Day of the Vipers)