It happened September 3, 1967.
The change over, also known as “Dagen H” in Swedish, was widely unpopular. One referendum after another, the proposal had been voted down over the previous forty years. But the politicians ignored the public will and forced the measure on the people.
The politicians argued:
1) Sweden’s immediate neighbours drove on the right, including Norway and Finland, with which Sweden has land borders.
2) Most Swedes drove left-hand drive vehicles. This led to many head-on collisions when passing on narrow two-lane highways.
Some 360,000 street signs had to be switched nationwide, which largely took place on a single day before the move to right-hand driving, with council workers joined by the military and working late into the night to ensure the task got done before H-Day formally revved into gear on Sunday morning. All but essential traffic was banned from the roads.