Iceland’s independence didn’t happen on June 17. It is just celebrated on that day. Great fun day in Iceland for sure!

“Iceland’s National Day, June 17, is celebrated every year to commemorate the establishment of a republic in Iceland in 1944. Before that time, however, the day also had a place in the hearts of Icelanders, because June 17 was Jón Sigurðsson’s birthday (1811-1879), Iceland’s main hero of independence. The day was chosen as a national holiday to honour his contribution to the Icelandic struggle for independence from Denmark.” https://visitreykjavik.is/national-day-iceland