AFAIK, It's a matter of changing your regional settings. The USA setting defaults to Sunday being the first day of the week. However, other countries use different defaults. All I can suggest is going to your Settings and find a region that starts in Monday (I think the UK is one...). Do keep in mind, however, that if you change the region settings - you'll get that region's defaults.
Regardless, to change it, navigate to General > International > Region Format.
Marilyn