Holidays that you add to your Holiday List (Settings > Holidays) will not immediately take effect on your schedules.
After adding or removing a holiday from your Holiday List, you must re-publish your project schedules for the changes to take effect.
Publishing in this way will completely update your schedule to account for the holiday. Until you re-publish your schedule, your trade partners and team members will not receive updated notifications for any date revisions on your schedule tasks.
Remember that holidays are date specific - so you'll need to enter the holidays that you observe for each calendar year.