Eh, that doesn't really answer it. Months are about 30 days long. First, there are better numbers than 7 to fit in 30. Five 6 says weeks would work much better for this. Second, moon "phases" are completely arbitrary. Really there are only two "phases", waxing and waning; anything else is arbitrary subdivisions.

The real reason there are seven days in the week is because there are seven moving objects in the sky. (From a human perspective.) There are seven days of the week:

1. The Day of the Sun
2. The Day of the Moon
3. The Day of Mars
4. The Day of Mercury
5. The Day of Jupiter
6. The Day of Venus
7. The Day of Saturn

These are the seven days of the week. They're named for the seven great moving lights in the heavens. Everything else is a fixed star or invisible to human eyes.