Check here: https://connect.garmin.com/modern/settings
"First Day of the Week"
Golly. I have had Garmin watches since 2003. And just got the new Epix Pro and noticed my run widget was using Sunday as the start for my weekly run total. Somehow, I had never changed this on any watch I have owned. Here I find a thread from 9 years ago with the answers. Love this community. Thank you!
(And confirmed that this is still the right menu location for the Epix and I bet the latest Forerunners and Fenixes.)
Many reports are for the past 7 days. Not calendar weeks. So, tomorrow it will be from Monday to Sunday.
On a related note, if you really want to see reports for the calendar week/year when they currently only display the last 7 days or last year, you can temporarily change your the date on your device and it will affect the start/end date of the report. So if I wanted to see a report for the last calendar year, I could change my PC's date to December 31 and view the report.
It would be nice if there were options for calendar week/month/year tho.