Egypt is notorious for its baking hot weather, particularly in the summer months (June - August), with many major cities reaching temperatures in the high 30s or above (Celcius) and little rainfall all year round. Most travellers avoid the peak heat and choose to travel between October and March. The weather remains temperate, making sightseeing trips to the Pyramids, temples and bazaars much more pleasant. However, everyone has the same idea and tourist numbers are high, meaning the hotel prices follow suit.
Visiting in the shoulder seasons (May and September) can be favourable as the weather remains milder than the summer time and room prices haven't reached peak season levels. In September, the sea breezes are lovely if you're planning a coastal visit and you'll enjoy the Red Sea without the usual autumn / winter crowds. Between April and May, the Khamsin Wind blows through Egypt, making conditions less ideal for photography. If you hope to visit during Ramadan, which falls between the same months, be aware that opening times often change and the cities are increasingly crowded.