I'm setting Sunday as my first day of the week, but after the new interface for Garmin Connect app it seems that the app read Monday as first day of the week.
I'm using android smartphone
In the app, tap your profile icon (top/left) then Settings, User Settings, First Day of the Week, select Sunday.
If for some reason that doesn't work using the app, you can also do it using the Garmin Connect Web site, under account settings, display preferences, First Day of the Week - https://connect.garmin.com/modern/settings/displayPreferences. Then restart the app.
See Garmin support doc - Can I Change the Day of the Week My Intensity Minutes Begin?
You cannot change the day of the week your intensity minutes begin. They are accumulated weekly from Monday–Sunday. Changing the First Day of the Week setting in your Garmin Connect account settings only applies to your calendar and related features.