Caesars Palace is planning a gala birthday bash next month to honor Las Vegas' most senior member of the entertainment set, comedian George Burns. Burns will celebrate his 95th birthday at a private party for invited guests Jan. 18 and a public performance Jan. 19. Until then, the entertainer, born Nathan Birnbaum on Jan. 20, 1896, isn't exactly sitting around counting his birthday candles. He appears several times a year at Caesars' resorts in Las Vegas, Lake Tahoe, Nev.; and Atlantic City, N.J., has just released his eighth book, "All My Best Friends." He'll be taping a television special next month to mark his birthday, and a fourth "Oh, God!" movie is being written for him. The new Burns book includes anecdotes about Eddie Cantor when he was a singing waiter in a Coney Island saloon; Milton Berle as a boy in a shoe ad; and the Marx Brothers when they were a musical act.