Brazil is a very large country with a warm tropical climate and varying weather patterns, so it can be visited all year around.
The hot summer months from October to March are the peak season in Rio de Janeiro, with the Carnival taking place 40 days before Easter (ie February or March) when prices rise significantly. However these months can be very hot and humid, so many people feel it is better to visit Rio during the cooler winter months from April to August when daytime temperatures are still pleasant, although cooler at night. Although weather patterns can be unpredicable nowadays, it tends to rain in Rio in August and September.
The best time to visit the Pantanal is from April to October (the dry season), when water levels are lower and animals congregate around the shrinking lagoons. Jaguar sightings are best at this time of year.
The Amazon rainforests are hot, wet and humid all year round. If taking a river cruise, the high water season is from December to May and the low water season is from June to November.
Salvador, Recife and the northeast coast have warm tropical weather all year round. Their rainy season from April to July, but usually only in the form of afternoon showers which quickly pass.