On September 3, 1967, all traffic in Sweden switch from driving on the left-hand side of the road to the right. Known as Dagen H (H day) or “Högertrafikomläggningen” (the right-hand traffic diversion), the change was initially very unpopular and had been repeatedly voted down during the previous 40 years. [source]

The arguments for making the change were substantial, including:

– All Sweden’s immediate neighbours drove on the right (including Norway and Finland, with which Sweden has land borders)
– Most Swedes drove left-hand drive (LHD) vehicles, which led to many head-on collisions when passing on narrow two-lane highways

 

According to Wikipedia:

Trams in central Stockholm were withdrawn and replaced by buses and over one thousand new buses were purchased with doors on the right-hand side. Some 8,000 older buses were retrofitted to provide doors on both sides. The modification of buses, paid by the state, was the largest cost of the change.
 
On Dagen H, all non-essential traffic was banned from the roads from 01:00 to 06:00. All vehicles had to come to a complete stop at 04:50, then carefully change to the right-hand side of the road and stop again before being allowed to proceed at 05:00. In Stockholm and Malmö, the ban was longer — from 10:00 on Saturday until 15:00 on Sunday. [source]