One of the biggest if not the biggest festival celebrated in Korea is Chuseok. A harvest festival held on the 15th day of the 8th lunar month which this year falls on September 12. This is a three day celebration where Koreans will visit their ancestors graves and there ancestral homes, leaving offerings and cleaning the graves. The families will also get together to enjoy a harvest meal as many cultures do at this time. There also may be contests like archery and a style of wrestling from Korea.
The festival is tied to the moon with a traditional dance done by the women and children under the moon light, Ganggangsullae
Chuseok Dance-1 Some rights reserved by geekgurl7
Also the food that is eaten at this time is Songpyeon and is shaped like the moon as well.
Image gotten from http://savstrom.wordpress.com/2010/09/17/i-will-eat-songpyeon/
So enjoy this festiv time of year and Happy Chuseok!
Thank you to Ask a Korean! for his help with this and if you have further question about Korea take a look at his sight and maybe even ask a question.
Other links:
http://en.wikipedia.org/wiki/Chuseok
http://english.visitkorea.or.kr/enu/SI/SI_EN_3_6.jsp?cid=811650