`THE DAY OF FIVE BILLION,' A TELEVISION DOCUMENTARY, airs tomorrow (WTBS, 10:15, EDT) in honor of the 5 billionth person born. Show includes interviews with world leaders and authors, plus musical performances by Peter Gabriel, Stevie Wonder, Nona Hendryx, Jos'e Feliciano, Jimmy Cliff, Youssou N'dour, Nana Vasconcelos, the Neville Brothers, and others. It will be beamed by satellite to reach virtually every country. THE LAKE GEORGE OPERA FESTIVAL in Glens Falls, N.Y., opens its season tonight with Verdi's ``Rigoletto.'' Gilbert and Sullivan's ``The Mikado'' starts next Friday, and Donizetti's romantic comedy ``The Elixir of Love'' begins July 21. Highlighting the summer season is the world premi`ere of ``Hazel Kirke'' (Aug. 7), adapted by Francis Cullinan from Steele MacKaye's 1879 melodrama. Performances are at the Queensbury Festival Auditorium.