The Bansko International Jazz Festival runs for a week every August, with free open-air concerts on the main Varosha square in front of the Holy Trinity bell tower. Running since 1998, it has hosted Joshua Redman, Bobby McFerrin, Branford Marsalis and a long list of European and Bulgarian acts, and it draws a crowd that more than doubles the town's population for the week.
The concerts are properly free; you turn up, stand or sit in the square, and stay as long as you like. Bansko's restaurants and bars stay open late. Hotel prices spike sharply for the week, so book ahead if you want to combine the festival with summer hiking.