To add holidays of the specified country into the default calendar in Outlook, please do as follows:

1. Click File > Options to open the Outlook Options dialog box.

2. In the Outlook Options dialog box, click Calendar in the left bar, and then click the Add Holidays button in the Calendar options section. See screenshot:

3. In the popping Add Holidays to Calendar dialog box, please check the specified country whose holidays you want to add, and click the OK button. See screenshot:

4. After importing, a Microsoft Outlook dialog box will come out as below screenshot shown. Please click the OK button successively to close all dialog boxes.

So far, all holidays of the specified country have been added to the default calendar of the default email account in Outlook already.

One click to remove (or restore) all birthday reminders in Outlook

With Kutools for Outlook's Remove Birthday Reminders feature, you can easily remove all birthday reminders from your Microsoft Outlook with only one click. Kutools for Outlook also provides the Restore Birthday Reminders feature to quickly restore all deleted birthday reminders with one click.

As we mentioned above, the Add Holidays feature can add holidays to the default calendars. However, if you need to add holidays to the second or others calendars, please achieve it with below steps:
Note: This method requires you have added holidays to the default calendar in Outlook.

1. In the Calendar view, click to open the default calendar, and then click View > Change View > List. See screenshot:

2. Click View > Categories in the Arrangement group to sort all calendar items by categories. See screenshot:

3. Select the category name of Holiday, right click, and select Move > Copy to Folder from the context menu. See screenshot:

4. In the opening Copy Items dialog box, please select the destination calendar you will copy the holidays to, and click the OK button.

5. Now a warning Microsoft Outlook dialog box comes out as below screenshot shown. Please click the OK button to go ahead.

Now all holidays are copying to the destination calendar in bulk. If you can add items in public calendars, you can also copy the holidays to the public calendar with the same way.

Remove duplicate public holiday entries in Outlook