July and August are by far the busiest months — Budva's Slovenska Plaza, Becici, and the Sveti Stefan area fill up quickly from mid-morning, and premium sunbeds at popular beach bars often sell out by 10 am. Prices for loungers and food are at their highest, and traffic on the coastal road can be slow.
June and September are noticeably quieter. You can usually walk up to most public beaches and find a spot without arriving at dawn. Beach bars are still open, the weather is sunny, and the sea remains very swimmable. Local Montenegrins tend to take their own summer holiday in August, so June and September also have a more relaxed, local atmosphere.
If you want reliably uncrowded conditions, aim for the first two weeks of June or the second half of September. Bear in mind that some smaller beach bars and boat taxis to remote coves (like Zanjic on the Lustica peninsula) may have reduced frequency outside peak season.