>>I have never heard of 7 being a lucky number in western culture and your link doesn't support that. 3 is a lucky number, 13 is an unlucky number, 7 is nothing to me.
Some sustain that 7 is the God's number, stemming from "God created the world in seven days"[1]
Also, _"According to some, 777 represents the threefold perfection of the Trinity"_ [2]