It is best to check the weather when choosing to go to Cairo because the climate can make vacationing rather uncomfortable. Tourists flock to Cairo between November and February when the weather is the coolest with an average temperature of 10-20°C (50-68°F). However, during this period hotel and airline prices take a leap, so if you want to avoid the herd of tourists or not want to be charged exorbitantly, it is best to hold off until September or October. During these months the weather is pleasant (and city is less populated) with highs of 30°C (86°F) and lows of 15°C (59°F). The least populated months are in the summer, between June and August, when the weather becomes unbearably hot with average highs of 34°C (93.2 °F). Also, be advised that sandstorms tend to occur between March and May, so it can get quite unpleasant during these months, especially if it rains suddenly and you get to experience the muddy streets.
However, the weather is not the only variable to consider when making your itinerary. The holy month of Ramadan should also influence your travel plans. For instance, Ramadan is the holiest month in Islamic faith, and therefore fasting takes place from sunrise to sunset. As a result, you will not find people eating, drinking, or smoking in public. And although such things are not totally prohibited, it is seen as rude, and you will be better off avoiding such indecencies. Also, most cafes, restaurants, shops, and other venues tend to close very early, so it can put a pullet in your travel plans. Then again, night time can be quite a spectacle, since the city comes to life again. During night time, when people are no longer fasting, feasts abound!
Ramadan will start in the last week of August in 2009.
Note: Though Cairo is warm all year long, be sure to bring a jacket and sweater if you plan on visiting the desert. Desert nights get quite chilly, though mornings are unbearably hot.