It really seems quite loony that the power to bring good luck should be ascribed to any mere number. In fact, loonyness-- the four parts of the lunar cycle, to be specific--is actually at the root of this number's supposedly magical powers. The phases of the moon are each about seven days long. So the number seven seemed to be embedded in the natural order of things, and somehow related to the heavenly body that has often been observed to powerfully affect the course of human events.

Hey, sure, the rainbow has seven colors. But when it comes down to it, what counts the most is that the week has seven days, and the seventh day of the week is also something really special. Wouldn't you know it: in luck as in much of life, it's all about weekends!

(Source: THE HANDY SCIENCE ANSWER BOOK)