If you forgot to select a public holiday group, or you created a public holiday group after the public holiday was added to your calendar, you can still assign a public holiday group to it retrospectively.
Assign a public holiday group to an individual public holiday
If there are not many public holidays that you want to assign a public holiday group, the easiest way to do this is to edit each relevant holiday individually.
Click your name in the top right, then click Settings.
On the settings menu click Calendar, then click Public holidays.
Next to the relevant public holiday click the pencil icon.
Under Holiday groups at the bottom, select the relevant holiday group(s).
This is now updated.
Assign a public holiday group to multiple public holidays
If you have multiple public holidays that you need to retrospectively assign a new public holiday group to, you may find it it easier to import your public holidays again.
When you follow those steps again, make sure you select all the relevant public holiday groups.