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" (lit. 'the right-hand traffic reorganisation'), was the day on 3 September 1967, in which the traffic in 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 traffic".
It was by far the largest logistical event in Sweden's history.