From the following passages it seems clear what was meant by the word days:
15. "Six days may work be done; but in the seventh is the Sabbath of rest, holy to the Lord: whosoever doeth any work in the Sabbath day, he shall surely be put to death."—Served him right!
16. "Wherefore, the children of Israel shall keep the Sabbath, to observe the Sabbath, throughout their generations, for a perpetual covenant.
17. "It is a sign between me and the children of Israel forever; for in six days the Lord made heaven and earth, and on the seventh day he rested and was refreshed.
18. "And he gave unto Moses, when he had made an end of communing with him upon Mount Sinai, two tables of testimony, tables of stone, written with the finger of God."—Exodus xxxi.
12. "Then spake Joshua to the Lord in the day when the Lord delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon, and thou, Moon, in the valley of Ajalon.
13. "And the sun stood still, and the moon stayed, until the people had avenged themselves upon their enemies. Is not this written in the book of Jasher? So the sun stood still in the midst of heaven; and hasted not to go down about a whole day.
14. "And there was no day like that before it or after it, that the Lord hearkened unto the voice of a man: for the Lord fought for Israel."—Josh. x.
These passages must certainly convey the idea that this world was made in six days, not six periods. And the reason why they were to keep the Sabbath was because the Creator rested on the seventh day—not period. If you say six periods, instead of six days, what becomes of your Sabbath? The only reason given in the Bible for observing the Sabbath is that God observed it—that he rested from his work that day and was refreshed. Take this reason away and the sacredness of that day has no foundation in the Scriptures.
WHAT IS THE ASTRONOMY OF THE BIBLE? IV.