Printing all calendars and design all calendar software with Sunday in the first column is a big clue that most Americans consider Sunday the start of the week.
In Europe, printed calendars and apps have Monday in the first column.
Being European, I prefer "Sunday in column 1" calendars.
Not for religious reasons or because I consider Sunday to actually be the first day of the week, but for practical reasons: It gives me a clearer understanding of the workweek. It moves Wednesday (in my language called "middle of the week") into the week's center. It deemphasizes the weekend for planning (and thus for planning work), making them better rest days.
In Europe, printed calendars and apps have Monday in the first column.