Herceg Novi has Montenegro's mildest climate, so the season stretches longer than most of the coast.
May, June, September and early October are the sweet spot — warm enough to swim, not so hot that the old-town staircases become a slog, and the crowds are thin compared to Kotor or Budva.
July and August are reliably hot (low to mid-30s) and busy. Accommodation prices spike and the promenade gets crowded in the evenings, but the boat trips to Žanjic and the Blue Cave run on full schedule and the open-air cinema in the Forte Mare is in full swing.
Winter is unexpectedly pleasant. The mimosa trees that line the seafront bloom in early February — there is a Mimosa Festival every year — and daytime temperatures often reach the mid-teens. The Kanli Kula closes; most restaurants stay open.