The first Vierdaagse, or four-day march, was held in 1909 as a way of keeping the army fit after the introduction of motor vehicles. Now mostly civilians take part, walking 30, 40 or 50 km a day over a period of four days. There are all sorts of variants on the theme, such as the Avondvierdaagse, which involves walking in the evening and is mainly done by primary school children. The biggest march is in Nijmegen and starts on the third Tuesday of July. It attracts some 45,000 walkers.