Is there any time of year to avoid Copenhagen? I'd say broadly no but it almost goes without saying that the winters here are dark and cold.
Summer - white nights and festive frolics
Copenhagen really comes into its own in summer. Gloriously long days, mild weather and a packed calendar of festivals and events put a smile on everyone's faces. It's also much more pleasurable to get around by bike when the weather's warmer. May to September is when there's most going on and all the attractions are open. Picnics in the parks are a pleasure and you can even hit the beach or one of the city's harbour pools if the water has warmed up enough - a reasonable bet at least in late summer as long as you're not expecting tropical waters.
The festival calendar is at its busiest during this time and includes dance music festival Distortion (www.cphdistortion.dk) held in venues throughout town in June, the Copenhagen Jazz Festival (www.jazzfestival.dk) in July, and the Pride festival (www.copenhagenpride.dk) a huge, gay party in early August.
One thing to consider is that Copenhagen can actually be relatively quiet in August as many people head for their rural summer houses. Some restaurants and other businesses close during this time but it also means you can usually get some competitive hotel deals too.
Autumn - culture and cuisine
There's still loads going on in Autumn with plenty of festivals and events packing the calendar and some interesting seasonal food at its best in the city's excellent restaurants - celebrated in style in late August during Copenhagen Cooking (www.copenhagencooking.dk). There are film (www.copenhagenfilmfestival.dk), art (www.artcph.com) and blues (www.copenhagenbluesfestival.dk) festivals in September, while in October Kulturnatten (www.kulturnatten.dk) sees every cultural venue throw open its doors all night. Entry is free and so is the public transport.
Winter - winter markets and cosy bars
The days are short and the nights long and dark and it can get really cold too. Having said that there are things to do in the darker, colder months. The Christmas Parade (last Saturday in November) through the city centre and the Christmas markets at Tivoli gardens are a festive draw and the city's many cosy bars and cafes make a lot of sense as hospitable havens from winter's clutches. There's a fair amount to do indoors too - from leisure centres to museums and galleries. In January there's the Winter Jazz Festival (www.vinterjazz.dk) and the atmospheric Night Film Festival (www.natfilm.dk).
Spring - back to life
In early Spring the city is still coming back to life and some attractions have yet to re-open. The Copenhagen Marathon (www.sparta.dk) and the massive, three-day Copenhagen Carnival (www.karnival.dk), both in May, kick the season off.