Employer's rights: employment contract and working hours

Questioner

My situation: I have been working at Albert Heijn for 7 years, with a 12-hour contract. I work these 12 hours on a weekday (6 hours) and on a Sunday (6 hours). In those 7 years I have also worked every Sunday with a total of 6 hours, every week. My employer has now shortened my working hours on this Sunday. He indicates that he wants to cut back (read: I am too expensive for Sunday). And that I am obliged to give up a wider availability, he is open to letting me work on other days. However, I want to keep this Sunday with 6 hours, what are my rights in this? Can I demand and/or get a lawyer to keep this?

Lawyer

Dear, in order to assess your rights and obligations, I would need to see your employment contract.