Answer:
For in six days the LORD made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the LORD blessed the Sabbath day and hallowed it (Exodus 20:11).
Our Bible verse tells us why God made the week seven days long. Let me explain. He made the seven-day week just for us. He created everything in six days and rested on the seventh day, right? Well, God could have made the entire universe in one split second because He is all-powerful and mighty. But He was setting a pattern for us to follow.
If we just kept on working every single day, we would get very tired and probably get sick. God was showing us that we can work six days, but then we need to rest for one day. Remember that He didn’t need to take that long to create everything (and He didn’t need to rest). Six days is actually a very long time for God. The seven-day week is for us!
Source: https://answersingenesis.org/kids/creation/why-did-god-make-a-week-seven-days-long/