Is working on Sunday a mortal sin?

Hi - My parents recently encouraged me to get a job, and I did at the local grocery store. After I was hired, I told the manager that I would really prefer not to work on Sundays. However, he has twice now scheduled me for Sundays. It is in the afternoon and only for 3 or 4 hours, so I don’t miss Mass. I still make time for God and family, and my parents really don’t want me to quit just because of working on Sundays. Am I being too scrupulous? It just seems like a grocery store isn’t exactly a necessary occupation on the Sabbath. I have to this Sunday, and I’m afraid it is a mortal sin.

These links should be helpful for understanding what the Church says about working on Sunday. If you have any further questions or concerns that are not answered by these links, please contact Catholic Answers directly.
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Is it OK to work on Sunday?**
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]* Can we volunteer to work on Easter if we need the money?**
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