Remember that when it's summer in the United States, it's winter in Argentina, and vice versa. Winters (July–September) are chilly. Summer's muggy heat (December–March) can be taxing at midday but makes for warm nights. During these months Argentines crowd resorts along the Atlantic and in Uruguay.
Spring (September–December) and autumn (April–June), with their mild temperatures are ideal for urban trekking. It's usually warm enough for just a light jacket, and it's right before or after the peak seasons. The best time for trips to Iguazú Falls is August–October, when temperatures are lower and the falls are fuller.