Evening temps can vary from 13-19 degrees by about 9pm. It’s bikini weather in the day, most hotels have heated pools & there are people in the sea; all trips are running as normal.
The resort & hotels are all beautifully decorated in festive mode & they make much of 24th & 31st Dec with Gala Dinners / buffets in the hotels. People tend to dress up a bit for this; especially New Year’s eve, when the hotels really go to town with the buffets. Some men wear dinner jackets, others wear jacket & tie. It's not compulsory but most people make an effort. Ladies can wear "sparkle", smart tops & smart trousers or long dresses; whatever is comfortable.
Local restaurants have special menus too. Hotels make more of an effort with the New Year entertainment on this night; normally hotel entertainment is quite dire. Xmas day is quiet & low key; an ideal opportunity to book a trip.
New Year is very busy in resort; more so than Christmas. Egyptians flock to the Red Sea resorts for 48 hrs or so to celebrate as well as many tourists.