Why Are There Seven Days in a Week?

The simple answer is because God created everything in six literal days and rested on the seventh.

The Fourth Commandment of the Decalogue is:
Remember the Sabbath day by keeping it holy. Six days you shall labor and do all your work, but the seventh day is a Sabbath to the LORD your God. On it you shall not do any work, neither you, nor your son or daughter, nor your manservant or maidservant, nor your animals, nor the alien within your gates. For in six days the LORD made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy.

There are some Christians who believe they have to try and manufacture "gap theories" and reinterpret terms like "days" in order to reconcile what the Bible says to the "scientific evidence" of a billions-of-years-old earth. However, the "evidence" they use originates from a faulty premise - a theory created to eliminate the need for God - and the only way to make it "feasible" is millions of years of random chance evolution. It takes a lot more "faith" to believe this implausible fairy story than the actual truth.

Truth is an absolute - any compromise with truth makes it untrue. If you know someone who's been honest with you the first ten times you've had dealings with him, but then he fudges the facts the eleventh time, he loses his credibility, doesn't he?

Take Ivory Soap, for example. It markets itself as 99 and 44/100% pure. To my mind, anything less than 100% pure isn't pure.

The Bible has been shown to be true 100% of the time when challenged, based on outside evidence - it doesn't need any help to make it agree with "science", and the stuff that can't be empirically proven, I'm willing to take on faith, since God hasn't lost credibility yet (and I'm betting eternity that He never will).

Here's an excellent article from the extremists over at Answers in Genesis on the length of the days of Creation.

Genesis 1:14-19
And God said, "Let there be lights in the expanse of the sky to separate the day from the night, and let them serve as signs to mark seasons and days and years, and let them be lights in the expanse of the sky to give light on the earth." And it was so. God made two great lights — the greater light to govern the day and the lesser light to govern the night. He also made the stars. God set them in the expanse of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good. And there was evening, and there was morning — the fourth day.

I will talk about how the sun, moon and stars could be created on the fourth day in a future post.
Isn't it interesting that from the point of view of someone on this planet, the sun and the moon are the same apparent size (just check out a picture of a total solar eclipse)?

Also, the stars are as reliable as clockwork (I enjoy watching Orion chase the seven sisters of the Pleiades across the sky every night). The planets, however, are a different story. They each seem to follow their individual agenda.

Check out this article on if the universe is so young, how can we see stars that are millions of light years away?

The Ancient Romans named the days after the five visible planets (which aren't part of the clockwork mechanism of the stars - the word planet means "wanderer") along with the sun and the moon. In fact, they also made up cool stories for the constellations, so they could "serve as signs to mark seasons and days and years".

The names they gave the days of the week still persist in "romance language" countries (romance is Latin for "in the Roman manner").

Latin: Dies Solis (sun), Lunae (moon), Martis (Mars), Mercurii (Mercury), Jovis (Jupiter), Veneris (Venus), Saturni (Saturn).

The English kept Saturday, Sunday, and Monday; but changed the rest to their more familiar Norse gods - Tiw replaced Mars, Woden - Mercury, Thor - Jupiter, and Freya - Venus.