South America covers such a huge geographical landmass that it is hard to generalize weather conditions across the continent as a whole. The further north you go, the warmer the climate and temperatures around the Equator are of course favorable all year round. Elevated locations in the Andes, such as La Paz and Cuzco tend to be much more temperate and thanks to the altitude in these areas you will need warm clothing at night no matter what time of year you visit. On the other hand, if you head to Pipa or Salvador in Brazil you should find no sleeves required, 365 days a year.
The dry season happens in Bolivia from May to October. In that time of the year daytime temperatures, even on the Altiplano, are warm and comfortable and there is little rain save for the odd flash flood.
Most South American countries have a long school holiday that runs from mid December to early February. Flight and other transport costs are likely to be higher at this time, as are hotel and hostel rates.
Try to consider what activities you are planning to take part in at each location. Aim to choose the dry season if you are thinking of hiking or doing other outdoor activities but bear in mind that any major waterfalls tend to be more impressive in the wet season when you can often reach them by taking a boat upstream rather than by air or wheeled transport.
If you are heading further south and don’t want to feel the cold, be sure to visit when temperatures are warmer. Santiago de Chile and Córdoba in Argentina make delightful spring destinations so October or November are a great choice. If you do find yourself there during the cooler months though, for winter sports enthusiasts the ski and boarding opportunities offered in the Andean resorts from May to September can make the visit quite compelling.
Brazil is such a large country it covers almost the whole spectrum of weather conditions just on its own. Down in the south, cities such as Florianapolis have a distinct winter from May to October but come into their own in the run-up to Christmas as the temperatures rise and the holiday season kicks in. Head to any beach destination in Brazil over the festive season and you can look forward to spending Christmas day supping caipirinhas on the sand. Be sure not to leave before carnival, happening every year in the run up to Ash Wednesday.