Yes, you can get national holidays through the calendar "Korean Holidays".
First you can list all the calendars by requesting below and then find the ID of the Korean holiday calendar.
GET /users/{id | userPrincipalName}/calendars
The calendar can be obtained by sending the following request with the ID.
GET /users/{id | userPrincipalName}/calendars/{id}
All events in the calendar can also be obtained by requesting the following:
GET /users/{id | userPrincipalName}/calendars/{id}/events
See the link below for more details:
https://learn.microsoft.com/en-us/graph/api/user-list-calendars?view=graph-rest-1.0&tabs=http
https://learn.microsoft.com/en-us/graph/api/calendar-get?view=graph-rest-1.0&tabs=http
https://learn.microsoft.com/en-us/graph/api/calendar-list-events?view=graph-rest-1.0&tabs=http
Hope this helps.
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