What happens on St George's day?

From the text:

“It is the eve of St. George’s Day. Do you not know that to-night, when the clock strikes midnight, all the evil things in the world will have full sway? Do you know where you are going, and what you are going to?” She was in such evident distress that I tried to comfort her, but without effect. Finally she went down on her knees and implored me not to go; at least to wait a day or two before starting.