Holidays are considered "non-working days" for purposes of your project schedules.
When CoConstruct calculates task dates based on predecessors and work days, it will skip over any holidays that have been entered on your account (Settings > Holidays), just as it skips over non-working days that you have specified for the project.
If you add a new holiday in the settings of your account, you will need to re-publish your schedule for the dates to reflect the new non-working day.
Holidays are date specific - so you will need to enter your observed holidays for each year.