September 3rd, 1967, Stockholm, Sweden: The day Sweden changed from driving on the left to driving on the right.
Mayhem.
On Dagen H, Sunday, 3 September, all non-essential traffic was banned from the roads from 01:00 to 06:00. Any vehicles on the roads during that time had to follow special rules. 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, however, the ban was longer—from 10:00 on Saturday until 15:00 on Sunday—to allow work crews to reconfigure intersections. Certain other towns also saw an extended ban, from 15:00 on Saturday until 15:00 on Sunday.
One-way streets presented unique problems. Bus stops had to be constructed on the other side of the street. Intersections had to be reshaped to allow traffic to merge.
Trams in central Stockholm, in Helsingborg and most lines in Malmö were withdrawn and replaced by buses, and over one thousand new buses were purchased with doors on the right-hand side.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, while Gothenburg exported its RHD buses to Pakistan and Kenya. The modification of buses, paid by the state, was the largest cost of the change. In Gothenburg and Norrköping, and in two Stockholm suburbs, tram networks continued to operate.
In order to avoid blinding the oncoming drivers, all Swedish vehicles had to have their original left-hand-traffic head lamps replaced with right-hand units. One of the reasons the Riksdag pushed ahead with Dagen H despite public unpopularity was that most vehicles in Sweden at the time used inexpensive, standardized round headlamps, but the trend towards more expensive model-specific headlamps had begun in Continental Europe and was expected to spread through most other parts of the world. Further delay in changing over from left- to right-hand traffic would have greatly increased the cost burden to vehicle owners.
Source: Wikipedia