Due to its size there are various times of year which are best to see the country as you will find small micro climates throughout.
The rainy season in the south of the country and in the Amazon is from December to April when you can expect heavy downfalls, though this is when it is warmest. During these months the north east of the country is hot and humid but much drier.
From May to November the country is generally dry and cool with the north east having a small rainy season from March to May. In the south of the country temperatures during the day average 22 degrees Celsius but inland and to the north of Rio they will reach the mid to high twenties with a little more humidity. From July to September is the best time to access the Pantanal but the Amazon River can become very low making certain lodges inaccessible.