Depending on the industry and/or company you're working in, it will or will not be possible for employees to register their trips to the office on public holidays.
When your account is set up, the standard setting will be to allow allowances during public holidays. However, if you do not want to allow your employees to register trips to the office during public holidays you can easily change this yourself.
Go to Settings
> Customization
> Scroll all the way down
> Unclick the toggle "Accept allowances during public holidays"
It's as easy as that!