This will depend a lot on where you want to go. You'll find wandering around Upper Egypt and Luxor uncomfortably hot in the summer months (June to August). Winter is definitely the best time to be in these areas. Summer is also the time when the Mediterranean coast is at its most crowded. However, winter in Cairo can get pretty cool, so spring and autumn are the best times to be there. Ideally, mid-May to mid-April would be the best time to enjoy the warm days without the crush of bodies on the beaches and the midday heat of high summer.