Paraty enjoys a round the clock summer. If you don't mind the crowds, the best time would be to visit during Carnival; book in advance though, or fall prey to the season higj rates. The Brazilian holiday season falls between Christmas and Carnival.
Weather wise, the ideal months would be between May and August. The sun is warm, winds are cool and temperatures remain between 22-32 degrees Celsius. Winter (June-August) is when the crowds start thinning and rates drop considerably; if you want to exploit the off season rates, visit during the monsoons (December-March), but remember to carry an umbrella with you!