I have sailed on Celebrity several times and never encountered any entertainment to pre-book.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Vision OTS 2-14-2015
Freedom OTS 3/6/16
Unless it's something very new I'm not aware of, they don't have any pre-booking for entertainment.
__________________ Travel Agent/Cruise Specialist w/15 yrs exp and 50 Cruises on 12 different cruise lines in many parts of the world! VIETNAM VETERAN OWNED & OPERATED. Certified Accessible Travel Advocate. Specializing in unsurpassed personal service to insure you have a hassle-free vacation!