In order to add holidays, go to Business Hours > Holidays and add a new holiday.

When adding a holiday, you have to set the start and end date. If your business is closed for the entire day during these dates, set Closed During These Period to Yes.

Otherwise, you can select No and you will be able to add custom opening hours for these dates.

How to add a holiday to set of business hours

Once you added a holiday, head to Business Hours > Business Hours and select the holiday you added under Holidays.