A country as long and thin as Chile is going to have a diverse range of weather patterns, but in short the country can be visited all year round with the exception of the far south.
The Atacama Desert in the far north of the country can be visited all year round as it is one of the driest places in the world. The Lake District and Patagonia are best visited between October and March. From April through to September Patagonia can become inaccessible and expect heavier rains in the Lake District during these months as well.
The below graph is based on Santiago where daytime temperatures average 25 Celsius but drop to an average of 7 at night.