The seventh day of the week according to the Shire-hobbits, coming after 'Mersday' according to their arrangements. Though the name is superficially similar to modern 'Friday', it was in fact a day of afternoon feasting and holidaying, and is therefore in fact rather closer to Sunday from a modern perspective.
Tolkien suggests that the Hobbits inherited their day-name from an older form, Hihdei, but he may well have been influenced by the fact that English contains a real term 'high day', which also refers to a day of festival and celebration.