I would say that Saturday and Sunday are relaxing days, not strong, because these days you can get out of the strong work routine

Saturday and Sunday are strong days, because all the others are weak days (week days).

I've never heard of it either. People who work Monday through Friday get Saturday and Sunday off, and do whatever they want on them, but they are not "strong".

I have never heard this. Why don't you post the whole joke?

I have never heard Saturday and Sunday referred to as 'strong'. Is this an American thing?