Weather in Bolivia varies with altitude rather than season.
The country sees two seasons, winter or dry season which runs between May and August and summer or rainy season which takes place November through March.
Springtime (September-November) is considered the best time to visit the country with sunny days and clear skies, still not too cold in the salt flats and the desert.
Summer or wet season (November-March) brings heat, humidity and mosquitoes, especially in the Amazon where roads can be impassable. However, rain is less frequent in the Altiplano and everything looks lush and greener.
When traveling to Uyuni during rainy season expect to have a unique experience of seen the salt flats reflecting like the world’s biggest mirror. Downside is that a tour to visit the Fish Island will not be possible because of water on the salar.