If you have created holidays in your settings this means your lessons created over that time period are made 'Inactive'. When a lesson is inactive it does not appear in your calendar and it is not billable.
If you still need to create a holiday find out how to do that here.
However; you may want to make one or more of those lessons active again if maybe you are running a one-off class or workshop over that holiday period.
To do this, head to the class that you want to make active. When you have clicked into the class details, head to the 'Lessons' tab. There you will see the 'Inactive' lesson created from the holiday you will have set in your settings.
All you need to do is press the 'Inactive' button on the lesson; it will make it 'Active' again.
Once it is 'Active', it will appear on your calendar and you will be able to bill your customers for it as normal.