Roskilde Festival (late June/early July) is the biggest event in the Danish calendar and one of the best music festivals in Europe. It runs for eight days in Roskilde, draws around 80,000 people, and has a serious commitment to non-commercial values despite its scale — profits go to charity. The lineup mixes global headliners with emerging acts. Camping on-site is part of the experience.
Copenhagen Jazz Festival (early July) takes over the city for ten days, with concerts in clubs, squares, courtyards, and concert halls. Much of the programme is free. The standard is high and the atmosphere is relaxed.
Distortion (early June) is Copenhagen's biggest street party — five days of outdoor raves, club nights, and neighbourhood parties. It migrates through different parts of the city each day.
Aarhus Festuge (late August) is a ten-day arts festival that transforms Aarhus with over a thousand events — theatre, music, art installations, and street performance, most of it free or cheap.
Sankthansaften (St. John's Eve, June 23) is Midsummer and widely celebrated across Denmark with bonfires on beaches and in parks, burning an effigy of a witch. It's a local summer ritual rather than a tourist spectacle.
Christmas markets (December) are at their best at Tivoli in Copenhagen and the old town square in Aarhus. The Tivoli one is particularly atmospheric — the park goes all-out with lights and rides. Buy a cup of gløgg (mulled wine) and take your time.