calendar-week-start-day is a variable defined in `calendar.el'.
Its value is

The day of the week on which a week in the calendar begins.
0 means Sunday (default), 1 means Monday, and so on.

If you change this variable directly (without using customize)
after starting `calendar', you should call `calendar-redraw' to
update the calendar display to reflect the change, otherwise
movement commands will not work correctly.

You can customize this variable.