Buenos Aires can be visited all year round but is best in the spring (October and November) when it is less hot and humid and the jacaranda trees are in bloom. In the winter (June to August) it can get quite cold.
Iguazu falls has a subtropical climate and can be visited all year round. It can be very hot and humid between December and April and slightly cooler the rest of the year.
Patagonia and the southern part of the country is best visited between October and April with the high season during the summer months of December to February. Although it is starting to turn colder in March and April, the autumn colours in these months are spectacular. It is important to note that the weather in Patagonia and the southern part of the country is very unpredictable even in high season and you can often expect to experience four seasons in one day! Over the winter (June to August) Patagonia is cold and dark, most hotels are closed and tours and services are not running.
Salta and the North West of the country can be visited all year round but receives less rain and more sunny days between April and December.
Mendoza and the wine regions can be visited all year round but a good time to go is during the harvest in the autumn months (March and April). The autumn colours in the Lake District at this time of year are also beautiful.
The whale watching season at Peninsular Valdes runs from mid-May to early December. Earlier in the season (May to September) the Southern Right Whales congregate in the bay around the town of Puerto Madryn and Playa Doradillo, but later on in the season they move further north around the peninsular and are more easily seen around Puerto Piramides.