The desert climate is harsh in both directions. May to September is the cool dry season — daytime highs around 20 to 25 degrees, nights often below freezing, very clear skies. This is by far the best time to walk. Carry a fleece for sunrise.
December to February is summer: daytime highs frequently above 40 degrees, the longer Kata Tjuta walks closed mid-morning, and afternoon thunderstorms not uncommon. Visits are still possible but plan everything for the early morning. The shoulder months (March, April, October, November) are pleasantly warm and quieter than peak season.