Argentina can be visited all year around, as it is a very large country with many different regional weather patterns.
Buenos Aires is best visited from October to March (the warm summer months) but the Winelands and Lake Districts are also lovely in the spring and autumn.
The Iguazu Falls have their highest flow in March and April, following the summer rains.
Patagonia is at its best from November to February, but being so far south the weather is changeable and most facilities are closed from June to August. The whale watching season is from May to December.