A system based on the number ten makes sense because that's the number of digits we have on our feet and hands. It's easy to keep track of things that way. But, seven is a mystical number that has roots in a place far far away long long ago called Ork. Ork, as we all know, has seven suns, and seven moons, and seven just seems to be the prime number built into their habitat. When the Ork-ians decided it would be fun to do a biology experiment and created us human critters in the Petri dish we know as planet earth, the number seven was built into our genes. Thus explaining the durability of the week as we know it today.