Weekly calendar shows next week if Saturday set as start of week

We only take appointments for Mon - Fri so I want to set the first day of the week (in my Wordpress General settings) as Saturday so that visitors to the site at the weekend see the availability for the coming week as default, rather than the past week.

The problem I have is if I select Saturday as the first day of the week then the plugin never displays a weekly calendar showing the current week. It defaults to showing the following week with no previous week button and no query string appended to the URL, as though the current week has already passed.

I've added a couple of screenshots showing the default appointments page generated with Saturday and Sunday selected as the first day of the week to illustrate what I mean.