To access the Season Types, click this icon:
What are Season Types? Basic groups for seasons
Where can you find them? Editors - Calendar
Season Types are used as a placeholder for seasons used by schedule and vacation.
The Season Type name can be anything.
For vacation, the name must correspond to the setup parameter "VacationSeasonTypePhase1". Otherwise, some parts of N-OC will complain about missing seasons, such as the Roster Wizard Vacation Balance will not work.