When planning your trip to Buenos Aires, remember that when it's summer in the United States, it's winter in Argentina, and vice versa. Winters (July-September) are chilly and rainy, though temperatures never drop below freezing. Summer's muggy heat (December-March) can be taxing at midday but makes for wonderful, warm nights. Spring (September-December) and autumn (April-June), with their mild temperatures -- and blossoms or changing leaves -- are ideal for urban trekking.