Foundation Day was a celebration in the Golden Empire, one of the three primary holidays on the Empire's calendar. It commemorated the founding of the Golden Empire by Rin Sakaros, Tariun Sakaros, and Keltrayu.
Since Rin intentionally coordinated the calendar to coincide with the months of the Galactic Standard Calendar, she chose to sign the Charter of the Golden Empire on one of the Republic's holidays, which was later replaced with Foundation Day in the Empire.
Celebrations of Foundation Day, aside from a day off work, were marked with fireworks, orbital displays, family gatherings, and various commemorations of the Empire's achievements.