Egypt is a year-round destination, but when you go matters a lot depending on where you are headed.
October through April is the best time for the Nile Valley and Cairo. Temperatures are warm and pleasant -- 20-25 degrees in Cairo, slightly warmer in Luxor and Aswan. This is peak tourist season, so expect higher prices and bigger crowds at major sites, especially around Christmas and New Year. December and January can be cool in the evenings, particularly in the desert -- bring a jacket.
Summer (June through September) in Upper Egypt is brutally hot. Luxor and Aswan regularly hit 40-45 degrees, and sightseeing in that heat is genuinely miserable. Cairo is hot and humid. If you must travel in summer, do your sightseeing early in the morning and retreat to air conditioning by midday. The upside is that tourist sites are nearly empty and prices drop.
The Red Sea coast has warm water year-round and is diveable in every month. Summer is hot but dry, and the sea is warm and calm. Winter brings slightly cooler air temperatures (still in the mid-20s) and is comfortable for both beach and diving. Hurghada, Sharm el Sheikh, and Dahab are pleasant from October through May.
Spring (March-April) brings the Khamsin -- a hot, dusty wind from the Sahara that can blow for days, filling the air with sand and turning the sky yellow. Flights can be delayed and visibility drops. It is not constant, but if it catches you, you will know.
Ramadan shifts dates each year (moving roughly 11 days earlier annually). During Ramadan, many restaurants close during daylight hours, and the pace of life slows during the day. After sunset, the iftar meal brings streets alive with food, socializing, and festive energy. Traveling during Ramadan is perfectly doable and can be a fascinating cultural experience, but be aware that opening hours and service may be affected.
Egyptian holidays to note: Eid al-Fitr (end of Ramadan) and Eid al-Adha are major holidays when domestic travel surges and internal transport fills up quickly. Book ahead. Sham el-Nessim, the spring festival celebrating the Monday after Coptic Easter, sees Egyptians picnic outdoors by the millions -- a lovely local tradition to witness.