Avoid - Going between June and August, the temperature is unbearable. Other than that, nothing to avoid.
Make sure you get to the temple at Karnak and a short cruise down the nile is nice.
I'd recommend doing Luxor as a twin centre with either Cairo or a red sea resort (if not both) so you get a bit of variation. I wouldn't spend longer than about four days in Luxor.
thanks Micky, I am going in two weeks and staying in Luxor just behind the temple as a base. I have a further question if you don't mind. What should women wear? I have just been told to be modest but does that mean no shorts?
If you do not mind the heat i would go in the middle of summer but visit the sites early in the morning because the light is much softer and it's cooler. If you are going from Cairo i would recommend getting the night sleeper train down, it's a great way to get there. You really need a good week just to see Luxor. Luxor temple, Karnak, valley of the kings and queens ect. Have a great trip. Rich, Photos of our travels around Egypt. http://www.franceinfocus.net/vlc6.html