The Welsh expression for Halloween is Nos Calan Gaeaf. Nos Calan Gaeaf is one of the Teir Nas Ysbrydion, Three Spirit Nights. The other two are the traditional Celtic holidays of Beltane (May 1) and Midsummer Night (June 21). These are the times when the veil between the seen and unseen worlds is at its thinnest, when spirits are most likely to break through.
One of those spirits, in Welsh folklore, is the hwch du gwta. She is a spectral black sow, a bad omen similar to the Grim in English folklore (famously appropriated by J. K. Rowling in Harry Potter and the Prisoner of Azkaban).
If you want to avoid visitors from the spirit realm, it's best to leave some bwyd cennad y meirw outside your door. Bwyd cennad y meirw is food for the dead.
Without a tasty bribe, you might get a visit from the Welsh equivalent of Ireland's sidhe, the fairies. The Welsh call them tylwethtag.
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