What are the rules about working on Sundays?

I have read a couple of times on these forums about a "2 hour rule " concerning necessary work on Sundays–that you could only do necessary work for 2 hours. Where did this come from? I have never been taught this, was I mis-instructed? Also, that doing unnecessary work is a mortal sin. Can you please comment on this? I am 65 years old, have been a Catholic all my life, and now I’m confused. Or are people being told wrong things and passing it off as true? Thank you.

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