Cultural events and popular festivals in public space is a creative change of the everyday urban space. Constituent streams of everyday life are changed for a period as the event take place.  Furthermore the bare change in social interaction during the festival raises questions to the taken-for-granted perceptions of time, space and other social categories. The festival is a heterotopic space, a space of abnormal normality. Therefore will popular festivals in urban space challenge physical planning; the organisation of planning, cooperation as well as fundamental principles in the planning system. Traditionally the Danish planning system has been focused on functional needs as providing housing, localisation of industry and infrastructure. Recently institutionalised culture has museums and sports arenas are given high priority, but neither in planning theory or planning practice is urban popular festivals an integrated element. Coping with festivals in planning are therefore hypothetic, ad hoc or concerned with practical questions as number of people in the streets, sound level or waste. On the basis of research into management and planning practice related to free popular festivals in public space on four different locations in Denmark and Sweden I discuss how festivals challenge the planning system, constellations of cooperation and especially how festivals contribute to renewal of the planning system.