October to April is the best time to visit. Daytime temperatures in this range are 20–28°C in the oasis and cold but manageable in the desert at night (bring a sleeping bag for White Desert camps). The winter nights in the desert can drop to near zero; stars are extraordinary in any season.
May to September is extremely hot — daytime temperatures in the oasis can reach 40°C, and the desert surface in summer is brutal. Some visitors still come in summer for the White Desert, arriving in the late afternoon and camping through to early morning before the heat builds. Ramadan changes the rhythm of the oasis towns considerably, with businesses closed during the day and active at night; not a bad time to visit if you adapt your schedule accordingly.