"Witches were active on the eves of St. George’s day and Midsummer. Then they were wont to go out and cut chips from doors and gates of the farmyard and boil them in a milk pail, and in that way charm the milk from that farm. Their plans might be frustrated, however, by carefully smearing the newly chipped places with mud."
[and I think that superstition was invented by a canny farmer as a way to get his chipped doors repaired immediately by the hired hands]
There is another superstition that Evil Spirits are abroad at midnight on St. George’s Eve, but that seems to be connected with his festival day in May.