New York City
I'm looking to travel to South America in the next year or so -- when are the flights cheapest to buy and on what airline?
Mentioned in this answer:
For internation flights, try booking about 12 weeks in advanced. This is when fares are usually cheapest. Google Flights is a great tool for price/airline comparison.
I don't know about the best time to purchase (other than to say that in general 2-3 months out are the best fares-but it is difficult to wait that long when planning overseas trips); however, typically Tuesday and Wednesday are the days the cheapest fares are offered. Early morning is best. United has a hub in Houston that offers non-top to many Central and South American cities. Plus Copa Airlines is a partner in the Star Alliance and they are a large provider in Latin America-so you can be on one ticket and travel seamlessly and easily.