Hvar's season runs from late May to early October. June and September are the best months: warm sea, sunny skies, full ferry timetables, and noticeably fewer people than in peak summer.
July and August are hot (often 32–35°C in the shade), crowded, and expensive. Hvar Town is at its most intense — also at its most exhausting. If you want quiet, head inland or to Vrboska and Jelsa.
The Lavender Festival in late June draws crowds to Velo Grablje for the harvest weekend. The traditional Procession of the Cross at Easter, when the cross is carried through six villages overnight, is on the UNESCO list of intangible heritage and worth timing a visit around if you are in Croatia in spring.
Outside the season the island is half-shuttered: most restaurants close, the catamaran services thin out, and a hard bura wind can blow for days. Atmospheric, but not the trip people come for.