In his declaration, Palpatine mandated that Imperial citizens would celebrate the anniversary of Empire's creation yearly, for the sake of posterity.
From same Wookieepedia article, on the celebration:
Empire Day was celebrated in a number of different ways, depending on region and culture. Although the holiday fell on one specific day of the year, celebrations in some corners of the Empire continued for weeks.
After the fall of the Empire in 12 ABY, was this holiday still observed galaxy-wide?