St. Patrick’s Day is celebrated in March. But the cast and crew of “Black Mass” are taking artistic license with the Irish holiday. Monday, we’re told, Scott Cooper, who’s directing the movie about South Boston mobster Whitey Bulger, will shoot a St. Patrick’s Day parade scene in Lynn. Extras have been told to arrive on set by 5 a.m. and to bring with them three outfits that recall early 1980s fashion. “NOT disco, ‘Saturday Night Fever’ costumes, etc. Just average, neutral clothes that may fit into that era,” reads the note from CP Casting. The movie based on the book by former Globe reporters Dick Lehr and Gerry O’Neill has been shooting in Southie this week, including a particularly grisly scene in the basement of a house on G Street. A source who was on the set said the scene involved the strangulation death of Deborah Hussey, played by Juno Temple, at the hands of Bulger, who’s played by Johnny Depp. “It was terrifying stuff,” said our source. We were also interested to learn that it was actor Peter Sarsgaard who played Bulger murder victim Brian Halloran in a bloody scene shot two weeks ago in Lynn. Sarsgaard, who’s best known for his role in “Garden State” and, more recently, “The Killing,” apparently took the role as a favor to someone. His name doesn’t appear on the list of cast members on IMDB.com.