Late spring and summer is the best time to visit Copenhagen. Sunny, breezy spring is about fewer crowds and lower rates. Warm season from June through September is full of popular events and festivals, such as Roskilde and Strøm. This also perfect time for exploring the Tivoli Gardens or watching deer in Dyrehaven. Autumn in Copen can be a pleasant time to cycle, but most of the attractions have reduced opening hours.
Although freezy winter is the period of city's hibernation, December is all about Danish festively decorated hygge streets, and hotels are full of tourists.