According to Wikipedia, Easter Saturday, on the Christian calendar, is the Saturday following the festival of Easter, i. e. the last day of Easter Week. However, the term Easter Saturday is also used colloquially to refer to Holy Saturday, the day after Good Friday and before Easter. As you can see in this picture ("Closed on Easter Saturday"), the owner of the shop made the same mistake, probably because Holy Friday, Easter Sunday and Easter Monday are bank holidays in Germany. So it is only understandable that most shops will close their doors on Holy Saturday as well.
But, however you call it and whatever you're doing this Saturday: Have a Happy Easter everyone!