The North West of Argentina (NOA in Spanish) is a frontier in more ways than one. Culturally it is where the largely European blood starts to mix with the indigenous Andean blood. Geographically it is where the seemingly endless plains of the litoral end and the Andes shoot up.
One might think that the best time to visit this area would be the winter when the temperature is milder, but in fact anytime of the year is excellent. If the weather turns sticky you can always do as the locals do and head to the mountains, where there is eternal natural air conditioning. Even then, as you work your way up to the Altiplano, you will still need a blanket or sleeping bag at night even in the heat of the summer.