The headline event is the Karlovy Vary International Film Festival in early July, one of the oldest and most prestigious film festivals in Central Europe, with a particular focus on Central and Eastern European cinema. The town fills with film industry people, critics, and cinephiles; hotel prices spike and the atmosphere is genuinely lively. If you are interested in cinema, this is when Karlovy Vary is at its most interesting.
The broader spa season runs from June through August, when the colonnades are busy with visitors from across Europe. The Russian connection that defined the town in the 19th century has a more ambivalent quality now, but the spa tradition itself remains intact and active.
Spring and autumn are quieter and pleasant. The spa woods are good for walking in all seasons, and the town is less crowded without being closed or dormant. Winter is quiet, some restaurants reduce their hours, but the colonnades are open year-round and a glass of Becherovka by a warm fire has an obvious appeal.