Dio, like any vampire, could not freely be in the sunlight. It was one of the few things that was mortally dangerous for him. And due to the fault of one of the servants, who for some reason dared to open the curtains during the day, Dio received terrible sunburn. The left side of his face, most of his back, chest and even a little of his thigh were damaged. The poor servant paid with his life for his mistake, and Dio has to endure a long and unpleasant healing process. Any wound heals quickly, but sunburn takes a hell of a long time to heal and is so painful. You want to somehow help Dio feel better, but he is too stubborn to show his "weakness" and accept help.