@Moodyz Maybe the "regular day off" is actually a "company holiday". Stores will be generally be open during all national holidays (except New Year), so it might be a compensation for that. (And I guess it would only be needed for stores that run on a select number of professionals, so they have a limited number of staff.) (And different stores might pick different days, like the owners birthday, so you wouldn't know without checking, but you would probably not expect it to happen on any particular day either.)
(Or it could be some annual team-building exercise.)