In later conventional use, by the time of the War of the Ring, Sterrendei had become shortened to Sterday. This is coincidentally similar to modern 'Saturday', and can be treated as a convenient translation. However, its function in the week was quite different from our own Saturday, and as the first of the seven week-days, a Hobbit would have seen it as more closely equivalent to modern Monday.
The only definite dating we have for the ancient day names of the Hobbits is in Appendix D to The Lord of the Rings, where we're told that they were in use 'at least nine hundred years before Frodo's time', hence the approximate date of III 2100 above. The fact that the words have a commonality with the language of the Rohirrim implies that their roots went back much farther, to the time when the ancestral Hobbits still dwelt alongside Men in the Vales of Anduin (though the precise form of the name Sterrendei probably underwent some evolution over that period).