May, June and September are the sweet spots — the sea is warm, the maquis still green in spring, and the town not yet at full bustle. July and August are hot, busy and expensive, but the Calvi on the Rocks festival in early July (electronic music) and the Festival du Jazz in late June draw a young, party crowd.
Winter is mild but the town empties out — many restaurants close from November to Easter. Pentecost brings the pilgrimage up to Notre-Dame de la Serra; the Rencontres de Chants Polyphoniques in mid-September fills the citadel churches with traditional Corsican choral singing for a week.