The best time to visit Rio de Janeiro.
Rio sits in the Southern Hemisphere, so the seasons run opposite to Europe and North America: summer is December to March, winter is June to August. It's warm all year — the city never really gets cold — so the real question isn't temperature, it's how busy and how expensive you want the trip to be.
The big picture: December to March is hot, lively and pricey, bookended by the two enormous dates of the calendar, Réveillon and Carnival. April–May and September–November are the shoulder — warm, calmer and the best value. June to August is mild, dry and the easiest time to move around the city.