Spring (April and May) and autumn (September and October) are by far the best times to visit. Daytime temperatures sit between 20°C and 30°C, the desert is in reasonable flower, and the migratory bird passage through the oasis is at its peak. Nights can be cold even in April, so bring layers.
Summer is punishing: temperatures regularly exceed 45°C in July and August, and spending time outdoors between 10am and 5pm is unpleasant at best and dangerous without adequate water. Winter brings frost and occasional snow, but the desert light in December and January is extraordinary — sharp, low, and intensely clear. If you can tolerate cold nights, a winter visit has a melancholy beauty entirely its own.