Traditionally Egypt has been seen as a winter destination, with most visitors arriving between October and April. At this time of year, Cairo and Lower Egypt (the area towards the Mediterranean) are pleasantly cool, with average maximum temperatures in the low twenties celsius.
Upper Egypt (around Aswan and Luxor) tends to be a few degrees warmer, but still very pleasant. There may be some rain in Lower Egypt, but it is light and sporadic.
The summer season from May to September is hot, especially in Upper Egypt where temperatures can hit 50C. The high temperatures are, however, alleviated by very dry air, airconditioning, far fewer visitors and lower prices, making summer a viable season for your trip.