The only viable season for visiting Djanet is October through March. Daytime temperatures in this window typically range from 20 to 30°C, dropping sharply at night — desert nights can be very cold, and sleeping outdoors on the Tassili plateau requires a good sleeping bag.
Summer (June to September) is brutal, with temperatures regularly exceeding 45°C and occasionally touching 50°C. No serious trekking is possible, and even short walks in the desert carry real heat-stroke risk. April and May are transitional — warm but manageable, though the heat builds quickly toward the end of May.
The festival of Sebiba, a Tuareg festival unique to the Djanet region, takes place in January or February (the exact date follows the Islamic calendar). It is an extraordinary event: days of costumed processions, traditional music, and ritualized mock combat between the two clans of the oasis. If your visit coincides with it, rearrange your itinerary to be there.