Semester holidays are generally those which schools take i.e Spring break, Winter break. National holidays are those which everyone takes i.e. Easter Monday, December 25th etc.,
Holidays for your country ( National holidays ) and your school ( Semester holidays ) are uploaded into your account when we create your school in ClassMaster. When you add lessons to any course in your school no lessons are created for National holidays. If, when you created the course, you ticked the Semester box then no lessons will be created for both National holidays and Semester holidays.