One of the fun and interesting things about living in a country with so much history (according to Korean textbooks, Korea has been in "existence" for over five thousand years..." that means that the culture is so deep rooted that old traditions are still upheld, even today.
One of these deep rooted traditions is the Korean holiday of Chusok, pronounced "chewsock", but quickly, so it doesn't sound like you are saying two words. As much as I would like to understand everything they are saying to me when they explain what happens during Chusok, I will do my best to describe what I know about the holiday. What you need to remember is that while Korea may be considered very religious now, historically, they were, and still are very traditional in their roots; most of the holidays are just celebrated by different religious groups, and not observed by others.
In Chusok, the preparations begin during the days leading up. What I have somewhat understood is that during this holiday of thanksgiving, you prepare large amounts of traditional food, for both those alive and those not alive. What's believed is that when you celebrate something such as Chusok, your ancestors who are no longer with you come into your house, and celebrate with you - so you're cooking not only for those alive, but those who are "joining" you from wherever they are.... Now most Christians, mostly just Protestants do not celebrate this holiday in the sense of cooking large amounts of food for those dead and alive, but they celebrate it more similarly to the North American style of being grateful to God for what He has given us.
Talking to my Korean friends, its quite the experience, in that its more work preparing for the holiday than it is to "celebrate" it. Chusok is always celebrated on the first full moon of September, and its the busiest holiday of the year. Last year, the holiday fell on a Wednesday in Korea, and the school had a two day work-week - Wednesday till Monday was a day off - this year, my luck, it took place on a Sunday - so we got the Monday off. My present roommate (I changed places where I'm living) booked his train ticket three months before the holiday, and was still only able to get a train that arrived back in Seoul at 12:30am, on the Sunday, one day before the actually event. The actual event takes place on Sunday, and its over somewhat quickly. Then, on the Monday, the whole city is pretty much a ghost town until the afternoon, when everyone who was away came home. The other part of Chusok that makes it so busy is that, no matter where you are living, you go back to your "hometown", where you were born. Think biblically, when there was an census, and it will make sense. For those who do not have a place to stay, you stay with relatives, which in Korea could be anyone.
Well, I hope this finds everyone in good health,
Take care,
Bryan