Argentina's climate is highly variable by both region and time of the year. One of the most popular regions to visit is that of Patagonia - during the cold winter months, almost everything shuts down. The climate in Patagonia can be unpredictable year round, but the best months to visit are from November to April, with January and February being the busiest. May to September in Patagonia sees winter climate and most hotels are closed (except those cateering towards skiing of course).
When the climate prevents travel to South Argentina, it's often the best time to visit the North, which is cooler and drier in the winter.
Buenos Aires' weather is extremely hot and humid during the summer months of December to February - make sure your hotel has air conditioning. Many locals leave the capital during these months, seeking a vacaion elsewhere in Argentina. Autumn and spring are better months to visit Buenos Aires.
The climate plays an important role in planning your trip to Argentina. In the summer months, it's best to travel south, in the winter months it's better to travel north. The huge diversity in the weather means that Argentina is a year round vacation destination, and can be visited during any month of the year.