There are many festivals throughout the year in Korea. Go to the website of visit Korea to check for specific dates.
I went to some festivals and here are some pictures.
Lantern Festival in Seoul, very cool!
I dont remember the nameof this, but it was light and music in Gyeongbokgung Palace
Fireworks in Seoul, I dont recommend this one. It was very crowded, people pushing.
Rice festival Icheon city
Very interesting and lot of makgeolli and food.
There is also Kimchi festival in Seoul, check it out^^