Hobbit week

The Hobbit week was taken from the dunedain and the names were translations of the names given by the dunedain following the eldar. The six days of the eldar already had days dedicated to the Sun,Moon,Stars, the two trees of Valinor, the Sky and the Valar or rulers, in that order being the chief or High day.

Origins
The Dunedain kept the dedications and order, but altered the forth day known origionally as tree-day with reference to the eldest tree of which a decendant grew in numenor, and desiring a seventh day week and being great mariners added a Sea-day after the sky day.

Hobbits week
The hobbits also took over this arangement, but the meanings of the days were soon forgotten and then reduced in form. The 'translation' was made more than a thousand years before bilbos time. In the oldest records in the shire in the earliest parts of the great writ of tuckborough, the names appeared in the archaic forms

Sterendai is starsday

Sunnendai is suns day

Monendai is moons day

Treowsdai is trees day

Heovensdai is heavens day or sky day

Meresdai is sea's day

Hihdei is high day

But in the red book of westmarch they are written as:

Stirday(or sterday), Sunday, Munday, Trewsday. Hevensday, Mersady. Hiday; and Hevensday was universaly pronounced hensday and often written He' nsday.