When you create a new Teamup calendar, the default start day for the week is Sunday. But you can change the default to any day of the week. Do so in Settings > Date & Time. Here you will see the option to choose the week start day as you wish. Keep in mind that this method requires administrator access to the calendar Settings, which is only available to calendar administrators.
If you are a non-admin user, you can not change the default setting, but you can temporarily change it by appending a link parameter to the calendar URL: weekStartDay=mo. See this example.