You can edit the first weekday in your settings.

This is used mostly to group your games more logically – e.g. if you want your Monday games to be grouped with the preceding weekend, you could change your First weekday to Tuesday.

This field becomes locked / disabled once you generate games. If you've already generated games, you will need to erase games, change your settings, and then re-generate your schedule in order to change it.

By default, all schedules are set to work with Monday-based weeks. If you want to change your default, you can do so in your organization settings – Edit Organization – at the bottom left.