I took the Discover South America trip in March a couple of years ago and the weather was wondeful. Most of the trip everyone wore light jackets or sweaters. Buenos Aires was the warmest, but there are only two days there at the end of the trip and it was comfortable, not hot and humid. Patagonia was the coolest and is unpredictable, definitely need warmer clothing just in case.
Summer is between December and February, Autumn is from March to May, Winter from June to August and Spring from September to November.
I asked our program director, who lives in Buenos Aires about the best time to take the OAT trip that goes to Cape Horn. He said March or November. Of course, that's farther south and will be cooler.