From the Kiwi-Yankee Dictionary by Louis S. Leland Jr.
isbn 0868681229

On the 30th July 1979, New Zealand introduced a drastic fuel conservation measure (in addition to banning petrol sales on weekends) This was the banning of operation of all privately and petrol powered motor vehicles weighing 4,400 lbs or less (except for motorcycles) for one day a week. That day to be chosen by the owner of the vehicle in question. There were several exceptions, public transport (taxis, buses, ambulances) rental cars an those (about 20% of vehicles) who have a good reason to drive on their carless day.

The introduction of thismeasure created some unexpected difficulties, e.g. the most popular days turned out to be Wednesday and Thursday, while the government had expected (and printed stickers for ) the weekend.