Summer (May to September) is the best time to visit. Temperatures hover around 20-22°C, the outdoor attractions are open, and the Veluwe forests are at their greenest. That said, rain is a constant -- expect at least 15 wet days a month even in summer, so bring an umbrella.
Winter is mild but grey. Temperatures rarely drop below freezing and snow is uncommon, but it rains more than 20 days a month and daylight is short. Most outdoor attractions close for the season.
The Jazz in the Woods festival in late May or early June is Apeldoorn's biggest cultural event, with open-air concerts spread across the Berg en Bos park. The Dragon Boat Festival is a long-running tradition with races and open-air performances. The Sunsation Festival in July brings poetry, theatre and music to the city around the summer solstice.