At Timeanddate.com, you can see the exact dates of all of Denmark's national holidays, observances, seasonal dates and when Daylight Savings Time starts.
On public holidays, the municipality offices and most, if not all, shops are closed. On observance days, closing times will depend on the office or shop.
You may note that some of the holidays relate to the official flag days. You can see the official flag days at Randers Municipality’s website (in Danish).