Why is the week seven days?
Question

                If Allah swt made everything in 6 days why do we have 7 days in a week?.
Praise be to Allah.Firstly: 
We cannot be certain about 
    anything with regard of the division of the week into seven days, or the 
    time when this division began – was it before the creation of the heavens 
    and the earth, or was the creation of the heavens and the earth what led to 
    this division? The reason for that is that we have no clear evidence which 
    states anything certain to that effect. But what is indicated by a study of 
    calendars and the customs and reports of different nations is that the 
    division of the week into seven days is connected to religion and taken from 
    it, and that it has to do with reports and rulings of religion. 
Shaykh al-Islam Ibn 
    Taymiyah (may Allaah have mercy on him) said in Majmoo’ al-Fataawa 
    (7/95): 
Every nation that does not 
    have a Book has no names for days of the week in their languages, rather 
    they have names for the day, the month and the year, because that may be 
    known on tangible and rational ways, so the nations gave them names, because 
    expression follows visualization. But the week was only known by hearing 
    (reports). It was not known that Allaah created the heavens and the earth 
    and everything in between in six days then rose over the Throne except by 
    the words of the Prophets who were enjoined to gather together on one day of 
    the week to worship Allaah and remember the first week in which Allaah began 
    to create the universe. In Arabic and Hebrew and the languages of those who 
    learned from them, there are words for the days of the week, unlike the 
    language of the Turks and others like them; there are no words for the days 
    of the week in their language because they did not know of that so they have 
    no words for it. 
Dr. Jawaad ‘Ali says: 
The month is divided into 
    four sections, and each section is a week composed of seven days. 
The idea of this division 
    is attributed to the Babylonians, but the days of the week and their order 
    in the manner that is known until now is a system that appeared long after 
    them. The week (shavu’ah in Hebrew) is mentioned in the Torah, in the Book 
    of Genesis. Based on both the Jewish Sabbath and the story of creation, the 
    week was organized along the lines that are still prevalent today. End 
    quote. 
Al-Mufassal fi Tareekh 
    al-‘Arab qabl al-Islam (16/96). 
And Allaah knows best.