From the beginning, time was divided into weeks, each consisting of six days of working and one of rest (Genesis 2:2,3; 7:10; 8:10,12; 29:28). The references to this division of days becomes afterwards more frequent (Exodus 34:22; Leviticus 12:5; Numbers 28:26; Deuteronomy 16:16; 2Chr 8:13; Jeremiah 5:24; Daniel 9:24-27; 10:2,3). It has been found to exist among almost all nations.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Week'". "Easton's Bible Dictionary".