Iceland got independence from Denmark in 1918 when Iceland got sovereignty through a deal with Denmark that stated that Iceland is an independent country. Iceland stayed a kingdom and made a deal with the Danish king that he be the king of Iceland. The Danes were to take care of the foreign policy of Iceland. The deal was signed on December 1st 1918.
In 1944 the Nazis had invaded Denmark. The Allies had ockupied Iceland already in 1940 and 1941. The king was in a house arrest and couldn’t take care of his job for Iceland. The deal with the king had expired so Iceland a vote to decide whether to stay in a personal union with the king or to found a republic with a president. Iceland became a republic in 1944.
The National Day of Iceland is June 17th because that’s when Iceland became a republic. But the Indepence Day is December 1st. Happy Independence Day!