Maybe start with a "What does Chuseok mean to me?", and relate it back to similar holidays where you're from, talking in a little more depth on that, and then follow that with a section of the lesson that has them answer the question, what does Chuseok mean to the students.
This way you model how they can talk about it, and then they talk about their own holiday.
You can finish with having an activity like the above comment dealing with traditional foods or portions of Chuseok.
Hope that helps!