December to March is the peak tourist season in Rio de Janeiro so it is best to book accommodation in advance (especially if you intend to go during Carnaval). Prices rise during this period and crowds are everywhere but this is Brazil’s festival season after all, so it is worth the bustle and noise. The low season is during the winter which is May to September (Brazil is on the other side of the equator, so summer and winter months are opposite from ours); this period is the cheapest and least crowded time to visit, with the exception of July which is school holiday month.
Humidity can be high in Rio though temperatures usually average thirty degrees. Rainfall is from October to January. Winter usually brings around twenty-three degrees with both rainy and clear days.