Dagen H in Sweden: Traffic changes from driving on the left to driving on the right overnight.

Dagen H (H-day), today usually called "Högertrafikomläggningen", was on 3 September 1967, the day on which Sweden switched from driving on the left-hand side of the road to the right. The "H" stands for "Högertrafik", the Swedish word for right-hand traffic. It was by far the largest logistical event in Sweden's history.