The ancient name in the Common Tongue for the 'Day of the Stars' that began the week. Its origins lay in the Elves' tradition of a six-day week, but by the time it came to be used in Middle-earth, the week had come to incorporate seven days. After Sterrendei, these archaic day-names followed the sequence Sunnendei, Monendei, Trewesdei, Hevenesdei, Meresdei and Hihdei.
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 days of the week, a Hobbit would have seen it as more closely equivalent to modern Monday.
Notes
1
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).
2
Strictly speaking, we only have direct evidence of the use of this old form in the Shire itself; for Buckland and Bree, we can only infer its likely usage. Buckland was founded in the Shire-year740, a point in time about two centuries after this old form of the name was recorded and so, at least in principle, it might already have mutated to Sterday before the first Bucklanders settled their land. In the case of Bree, we know that the newer 'modern' forms of the day names were used there, and given that Bree was older than the Shire, it seems a reasonable presumption that the older forms were also used there historically, but this not explicitly stated.