Don't even ask me to get into the mathematical term, I swear it's partly arbitrary. But lots of people over the years have considered 3 and or 7 to be lucky. I think it's pretty much because people want to have lucky numbers, but if you ask them for a reason they'll usually point to Biblical examples. If they're Westerners. Eastern cultures are more likely to point to the similarity between the sound of a number and the sound of a particular word.

OK, I like Rusty's answer better. She said the same thing, but I sound like an "I'm above all this" ass.