Narcissa Black does not study. Narcissa Black does not care for studying. Narcissa Black, however, is not allowed to fail classes. After a message to her parents from Professor McGonagall, she had been threatened and coerced into studying Transfiguration to the point that she actually might get an A on her OWL, if it were tomorrow. Unfortunately, it isn't, so she has to continue to work on it. She wasn't about to go a whole summer without buying a new wardrobe, and she wouldn't get one unless she got at least an A.
She was taking a break, though. It was Saturday. After taking the morning to learn the newest part (she could now make her snail fade quite a bit as opposed to not at all), she was going to spend this beautiful spring day outside with her friends. She could only study when they were away, or they'd make fun of her, or at least question why, and Cissa just didn't want to put up with that.
Either way, Narcissa Black was taking a study break, and going outside.