Aswan is at its best from November to March, when daytime temperatures settle into the comfortable low-to-mid 20s Celsius and the desert nights turn genuinely cool. December through February is the most popular window, with pleasant sightseeing weather but the biggest crowds and highest prices. Aswan is one of the hottest inhabited places on Earth in summer: from May to September, daytime highs regularly pass 40C and can exceed 45C in June to August, with barely any relief after dark. Visiting between June and September means planning everything around the two cooler windows of the day and resting indoors through the afternoon.
Aswan's heat is more extreme than Cairo's or even Luxor's, especially from May to September, when daytime temperatures regularly exceed 40C and nights stay warm. Schedule outdoor sightseeing for early morning or late afternoon, carry more water than feels necessary, and treat the hottest midday hours as rest time.
Boats offer little shade, and sun and glare off the water are stronger than they feel. Wear a hat, sunglasses and sunscreen on any felucca or motorboat trip, and keep small children and older travellers especially well protected and hydrated.
Dehydration is the main everyday risk for visitors, given the dryness and heat. Carry a refillable water bottle, drink more than you would at home, and watch for dizziness or headache as early warning signs, especially during the long Abu Simbel excursion.