I say this every year and every year there are some people still disappointed. December, especially the two weeks either side of Christmas Day 25 Dec are the busiest days of the entire year in Egypt.
If you are hoping to book your holiday for anytime in those 3 weeks you really need to confirm your flights and hotel/cruise reservations. There is always a shortage of flights, transport and accommodation at the higher end of the hotel and cruise sectors. This year there are even more going to be disappointed. Already there are days when there are no flights available at certain times between Cairo, Luxor and Aswan from mid-Dec to mid-Jan 2019.