Although demons, being spiritual beings, do not need food, according to Christian demonology demons, and especially the Devil hate salt, and thus no food is served with salt during the Sabbaths. Bread, if not made with rye, and oil, are prohibited. This is what the Christian church believed.
This can have an explanation in the fact that in some Christian rituals of baptism, especially in Catholicism, salt is put on the lips of the child during the ceremony of the baptism as a symbol of wisdom. Demons do not reject knowledge but perhaps dislike the religious symbol of it. The dislike for bread can be explained due to the fact that it represents the body of Jesus for Christians and is transubstantiated into his flesh during the mass.
Nevertheless, wine is the Christian symbol that during the mass is transubstantiated into the flesh and blood of Jesus and demons do not show dislike for wine. Some demons are said to be able to turn blood into wine and vice versa (some of them are mentioned in the Ars Goetia of the Lemegeton and in Pseudomonarchia Daemonum). During some time it was believed that offering bread and wine to a demon was an invitation to him to stay in that house and to possess that person; of course this could be without knowing that the incomer was a demon, so it was not convenient to offer those common thing to any foreigner.
It was believed that during the Sabbath the Devil could extract wine from certain plants, especially by making a cut in the trunk of a tree, and mixed it with his blood (maybe the like of demons for wine is due to this belief). This was probably due to the fact that Christian demonologists believed that during the Sabbath was celebrated a Black Mass with a parody of the communion.
Oil is another Christian symbol, being the substance with which Jesus was consecrated as Christ (from Greek 'Christos', anointed), and there are also demons that can turn blood into oil and so on, named in the same grimoires above-mentioned.
It was believed that during the Sabbath the attendants, or at least the most important ones, ate human flesh, which shared with the Devil, present in the meeting and presiding it.
