The first thing to remember is that the southern hemisphere seasons are the opposite of those in the northern hemisphere. Due to its length however, Chile houses a variety of climates at any given time which are influenced by the Pacific Ocean in the west and the Andes to the east, whilst the various altitudes and topographical reliefs throughout the country also have an impact on the climate and weather patterns. The Upshot is that if you carefully design your itinerary Chile can be a year round destination, though as a general rule the best months to travel are from October through to the end of April if you intend to travel to Patagonia.
The peak season across the country falls in the months of January and February, the Chilean summer holiday period and in theory the driest and warmest months of the year in Patagonia. At this time of year lodges and hotels in the south do fill up, and it is wise to book as far in advance as possible to secure your first choice accommodation. To avoid the busiest times of year, the slightly quieter shoulder seasons of October to December and March to April showcase Chile's spring and Autumn, and are fabulous times to be in Patagonia north or south. The turning of the leaves towards the end of the season, and the yellow and brown colours creeping into the landscape make for some beautiful scenes, whilst the winds are also often less fierce. In the crisp springtime meanwhile, Patagonia is starting to burst into the life, and the temperatures in Santiago are cooler and more comfortable than in the height of summer.
If you prefer to travel in the northern hemisphere spring/summer months then the north of the country is driest from April through to September. The Chilean Autumn is also the time of year for the wine harvest, making this an excellent window to visit the Central Valley. Lodges in Patagonia are often now still open and remain accessible until May where you can expect far fewer visitors in the national parks, and significantly lower prices if you are prepared for a taste of the Patagonian winter.