If I write in my application for a visa for Egypt that I will travel on a specific date, and I get the visa with a duration of 3 months but with single entry, does it mean I can enter any time (once of course) within that 3 months, or do I have to enter on the date I wrote in application? The same date is written on my flight tickets and hotel reservation, but I want to delay it now. Is it okay to do that?
You can enter at any point during the three-month period of validity.
With some countries, you must leave the country before the visa expires. With others, you only need to enter before the visa expires. If your delayed travel plans mean that you wish to remain in Egypt after the expiration date of the visa, you'll need to find out which kind of country Egypt is. If you plan to depart before the visa expires in any case, then you don't need to worry about this.