The honest short version: there is no wrong month on this coast, only different weather and different crowds. Here is the long version, so you can pick on purpose.
The seasons
Summer (December–March) is hot, green and full — Brazilian holiday season, afternoon storms that are their own entertainment from the veranda, the bay at its warmest. New Year and Carnival pack the town. Autumn (April–June) is the sweet spot: rain tapers, the sea is still warm, crowds go home. Winter (June–August) is the dry season — mild, clear days, cool evenings, the best footing for waterfalls and trails. Spring (September–November) is green, quiet and underrated.
FLIP
FLIP — the Festa Literária Internacional de Paraty — began in 2003 and grew into Latin America's best-known literary festival: five days of authors, readings and a historic center turned entirely into a book fair, with music at night. Dates move year to year, so check the current edition. You do not need to read Portuguese to enjoy it; the street is the ticket. Beds in town vanish months ahead — which is exactly when staying on a hillside ten minutes away, with parking for five cars, stops being a detail and starts being the plan.
The cachaça festival, and the sacred calendar
Every August the town throws the Festival da Cachaça, Cultura e Sabores — the region's distillers pouring side by side, food stalls, live music. Good-natured rather than rowdy. Around the year, the old religious festivals still run deep: the Festa do Divino around Pentecost, the patron saint's feast of Nossa Senhora dos Remédios in September, candlelit Holy Week processions on the stone streets.
Our recommendation
For a first stay: May or September — warm sea, dry trails, the old town unhurried. For festival energy: FLIP or the August cachaça weekend, booked far ahead. For a big family summer: January works beautifully if you plan boats for mornings and let the afternoon storms perform over the bay while the feijoada cooks in the gourmet pavilion.