Stockholm's nightlife can be broken up into two general groups based on geography. First, there's Birger Jarlsgatan, Stureplan, and the city end of Kungsträdgården, which are more trendy and expensive. At the bars and clubs in this area it's not unusual to wait in line with people who look like they just stepped off the pages of a glossy magazine. To the south, in Södermalm, things are a bit looser and wilder, but that doesn't mean the bars are any less hip. At night Söder can get pretty crazy—it's louder and more bohemian, and partygoers often walk the streets.
Many establishments will post and enforce a minimum age requirement, which could be anywhere from 18 to 30, depending on the clientele they wish to serve, and they may frown on jeans and sneakers. Your safest bet is to wear black clothes, Stockholm's shade of choice. Most places are open until around 3 am. Wherever you end up, a night of barhopping in Stockholm (and all of Sweden) includes fresher air now, since smoking has been banned since 2005 in all bars, clubs, and restaurants.
Stockholm's theater and opera season runs from September through May. Both Dramaten (the National Theater) and Operan (the Royal Opera) shut down in the summer months. When it comes to popular music, big-name international acts frequently come to Stockholm in summer while on their European tours. Artists of this type always play at Globen sports arena.
For a list of events pick up the free booklet What's On (www.stockholmtown.com), available from hotels, tourist centers, and some restaurants. It lists the month's events in both English and Swedish.