Hi @IBN
I found you have posted the same question in another thread. So I will give you our advice from your topic. If I misunderstand your feelings, please contact with us!
According to your topic, if you want to change the first day of the week in Teams calendar to Sunday, you could try to follow these steps to complete it:
1) Login to Outlook web App.
2)Check your first day of week in your setting like the following picture:
3)In Teams calendar:
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