Holidays are hardcoded to automatically enable; therefore, the default setting for a timezone does not have a time interval on a holiday. If a holiday is enabled, then the timezone will not function for the entire holiday.
To set an interval during a holiday, configure a timezone with an interval during the desired hours. Then and only then put a check on the holiday type that corresponds to the holiday. The check enables the interval for the holiday type selected. The holiday type is a group of individual holidays.
Timezones can be accessed through System Administration by going to Access Control | Timezones... | Timezone tab. Select Modify to edit an existing Timezone, or Add to configure a new Timezone. Holidays are configured on the Holidays tab in the same window.