At last, HBO has announced when it will air “Confirmation,’’ the much-anticipated movie about the 1991 confirmation hearings of Supreme Court Justice Clarence Thomas, whom Anita Hill accused of sexual harassment when they worked together at the Equal Employment Opportunity Commission. The film will debut April 16. Hill, who now teaches at Brandeis, is played in the film by actress Kerry Washington. “Confirmation,’’ directed by Rick Famuyiwa, costars Wendell Pierce as Thomas, Jeffrey Wright as Harvard prof Charles Ogletree, Treat Williams as Ted Kennedy, and Greg Kinnear as Joe Biden.