December to March is peak season: temperatures hit 30°C and above, the beaches fill with Brazilian and Argentine visitors, and prices rise sharply. January and February are the most crowded months — lively if you're there for the party, expensive and congested if you're not. New Year's Eve and Carnival (late February or early March) bring the biggest crowds and highest prices.
April to May and September to October are the best times to visit if you want good weather without the full summer circus. Temperatures are comfortable (22–26°C), the beaches are quieter, and accommodation is cheaper and easier to find.
June to August is the coolest period (18–22°C), some beaches are rough and a few beachside businesses close, but it's the best time for surfing — the south swells arrive — and for whale watching off the south coast. Windsurfers and kitesurfers favour August to November for reliable winds. Budget travellers should avoid January and February entirely.