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.
The 'Highday' of the Hobbits of the later Third Age developed from an older form, Hihdei. Tolkien's choice of the term may conceivably have been influenced by the fact that English contains a real term 'high day', which also refers to a day of festival and celebration.