Holidays that you add to your Holiday List (Main Menu > Settings > Holidays) will not immediately take effect on your schedules.
When you visit your schedules after adding a new holiday, you will find that the new dates are automatically recalculated, but the schedule will be in edit mode with options to save your changes at the bottom of the page.
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.