Travelling to Egypt and deciding when is really a matter of activities. If you are into swimming and snorkelling in the ocean, clubbing with sand between your feet and getting a real nice tan, then there is no reason not to head to Sharm el-Sheikh or Hurghada during the very hot months of summer where 38 - 45 degrees celcius are not uncommon.

If you are more into sightseeing and cultural stuff, then the best time to go is between October and April. Christmas and Easter are considered the very high season. No matter what the weather, the nights during winter can get cold so always wear layers. During the hottest months of summer, the nights are warm and balmy, but it can be really windy on the coasts.  The sun is extremely strong all year round so take a hat and have plenty of SPF or you will peel. 

Also remember that places in the south like Aswan and Luxor are hot during the summer. Egypt is close to deserts so there is always sand in the air so forget about wearing contact lenses.   Make sure you take your sun glasses. 

Be careful on hot days especially in summer.  If you want to see sites during hot days, either go early in the morning or late in the day.     Do enjoy a stroll along the Nile in the evening as there will be a cool breeze and lots of people watching going on or try a felucca ride.

Either season, the winds can be strong and during the early months of spring, there is a risk of severe sandstorms in Egypt, the khamsin. When the khamsin is blowing, you can do nothing but stay inside and wait until it has passed. Sightseeing is not possible during a sandstorm. Windy summernights are nice, but be careful during the day, as the wind sometimes fool you so that you do not feel the heat and the rays from the sun and you will burn easily.