Legends of Innis-Sark (4) - Legends, Charms, and Superstitions of Ireland

The islanders believe also that angels are constantly present amongst them, and all blessed things—the rain, and the dew, and the green crops—come from their power; but the fairies often bring sickness, and will do malicious tricks, and lame a horse, or steal the milk and butter, if they have been offended or deprived of their rights.

There are certain days on which it is not right to speak of the fairies. These days are Wednesdays and Fridays, for then they are present though invisible, and can hear everything, and lay their plans as to what they will carry off. On Friday especially their power for evil is very strong, and misfortunes are dreaded in the household. Therefore, on that day the children and cattle are strictly watched; a lighted wisp of straw is turned round the baby's head, and a quenched coal is set under the cradle and under the churn. And if the horses are restive in the stable, then the people know the fairies are riding on their backs. So they spit three times at the animal, when the fairies scamper off. This cure by the saliva is the most ancient of all superstitions, and the islanders still have the greatest faith in its mysterious power and efficacy.