Hebrew shabua', a period of sevens; Greek hebdomas. Is astronomically an appropriate division, as being the fourth of the 28 days' lunar month (more exactly 27 days, 7 hours, and 43 minutes). In Genesis 4:3 (margin) "at the end of days" the reference may be to such a period; but Abenezra explains "at the end of the year," namely, after the fruits of the earth were gathered in, the usual time for sacrifice. Noah's waiting other "seven days" (Genesis 8:10), and Laban's requiring Jacob to fulfill Leah's "week," i.e. celebrate the marriage feast for a week with Leah (Genesis 29:27), are explicit allusions to this division of time (compare Judges 14:12); also Joseph's mourning for Jacob seven days (Genesis 50:10). The week of seven days was the basis of the sabbatical seven years, and of the Jubilee year after seven sevens of years.
Pentecost came a week of weeks after Passover, and was therefore called the feast of weeks (Exodus 34:22). The Passover and the tabernacles' feast was for seven days each. (See SABBATH, on the beginning of this division dating as far back as God's rest on the seventh day after creation). It prevailed in many ancient nations; all the Semitic races, the Peruvians, Hindus, and Chinese. The Mahratta week has Aditwar (from aditya the sun, and war day), Somwar (from som the moon) Monday, Mungulwar (from Mungul Mars) Tuesday, Boodhwar (from Boodh Mercury) Wednesday, Bruhusputwar (from Bruhusputi Jupiter), Shookurwar (from Shookru Venus), and Shuniwar (from Shuni Saturn).
As Judah's captivity in Babylon was for 70 years, so its time of deliverance by Messiah was to be 70 sevens of years (Daniel 9:24-27). (See DANIEL.) Seven was a predominant number in Persia; seven days of feasting, seven chamberlains, seven princes (Esther 1:5; Esther 1:10; Esther 1:14). Rome adopted the division by weeks.
Fausset, Andrew Robert M.A., D.D., "Definition for 'week' Fausset's Bible Dictionary".