H-Day in Sweden also known as Hogertrafik Day. This is the day in which Sweden decided to switch from driving on the left to the right because there was an increase in accidents of Swedish drivers driving outside of the country. The mass car accidents abroad for their residents and visitors crashing was a problem since all neighboring countries drove on the right and not the left. In Europe today only the United Kingdom & Ireland still drove on the left but the rest of Europe. After advertising with a song specifically about the switch, billboards, pamphlets, radio advertisements, and specific rules set in place to make the switch it was set for 9/3/1967 for the switch from one side to the other.
Sweden was he LAST mainland European country to switch driving sides. Belgium and the Netherlands switched in the 1920's, eastern Austria (Yes half of Austria drove on one side of the road and the other half drove on the other. Eastern Austria drove on the Left and Western drove on the right), Hungary, and Czechoslovakia all drove on the left until Hitler invaded in World War II and changed that. Here in North America, Nova Scotia drove left until the 1920's, and Newfoundland until 1947. The US Virgin Islands still drive on the left and the closest area that drives on the opposite side of the road is....The Bahamas.
To learn more about Hofertrafik Day listen to this podcast: https://99percentinvisible.org/episode/h-day/