In the US we are used to seeing calendar weeks start with Sunday. I suggest you have an option to set which day to start with.
I noticed your app has no localization. I am in the US and all the dates are displayed in Asian format. Our week starts on Sunday instead of Monday. The calendar view is all wrong. Users constantly pick the wrong day on the calendar because yours starts with Monday instead of Sunday.
I’ve never used a Monday to Saturday week format so I agree, this would be helpful.