Don’t mess with Johanna Day. The actress slapped her “God of Carnage’’ costar Stephen Bogardus so hard during the fight scene of Saturday’s Huntington Theatre matinee that the evening performance had to be canceled at the last minute. According to Huntington spokeswoman Rebecca Curtiss, Day (above) was in so much pain after delivering the slap that she was taken to Mass General, where her hand was found to be broken. Day doesn’t have an understudy, so the show did not go on. She was back on stage yesterday but with a cast. Curtiss said the fight scene was choreographed again yesterday morning to accommodate the injury. (For the record, Bogardus is just fine. The slap wasn’t actually that severe - just awkward. “Her ring may have gotten caught,’’ Curtiss explained.) The Huntington was able to get in touch with many Saturday-night ticket holders before the performance, but about 100 to 200 people did show up for the show. Curtiss said the patrons were understanding and that about 15 of them stuck around to watch the Patriots-Broncos playoff game with theater crew members, who took advantage of the night off by projecting the playoff onto the theater scrim. Day’s friend Mary-Louise Parker, who’s in town to film the movie “R.I.P.D.,’’ was expected at yesterday’s performance.