How to configuration of the first day in a week

In terms of how to configure the calendar options, see

https://www.drupal.org/docs/extending-drupal/contributed-modules/contrib... →

The offical document about the option of first day in a week.

https://fullcalendar.io/docs/v5/firstDay

The first date behaviour is already supported out of the box based on the current Drupal's configured first day of the week settings (visit /admin/config/regional/settings in your site to check).

But if you want a value that's different to what's configured for the site, then you may override the firstDay option yourself using the Advanced settings section.