Battle of Heligoland Day is observed in Denmark every March 9, commemorating a naval engagement during the Second Schleswig War in 1864. The battle took place in the North Sea between a Danish squadron and a joint Austro-Prussian squadron off the coast of Heligoland, a territory then under British neutrality. The Danish forces, led by Commodore Edouard Suenson, achieved a tactical victory, suffering fewer casualties compared to their opponents. This day is recognized as an official flag flying day in Denmark, highlighting its significance in Danish naval history. The observance serves to honor one of Denmark's major naval victories and reflects the nation's maritime heritage.