Why did the ancients superstitious about the number "7"?

Is this 7 the Arabic numeral 7 in the west or the seven or seven in the east? This should be understood. Can't be confused. I just saw a great god quote many mysterious laws and numbers. But this is a coincidence. Let me start with some allusions in China legends, which are similar to the legends from 0 to 9. First of all, the avenue is extended for 50 days. Why is the sky extension 49? Because there is an escape 1, then Tai Chi gives birth to two instruments, and two instruments give birth to four images, and then it extends back. This so-called escape 1 may also be called radix. In ancient times, escaped 1 was used to describe the time when heaven and earth were not open. The so-called "9" does not mean the real "9" but an extreme expression, with 9 as the pole and 1 as the base. I don't quite agree with the answer from the downstairs analysis. If Avenue 50 is completed and there is a so-called return to the market, it is most appropriate to use a number to represent 0, but another 0 represents perfection. Don't ask me how I know.