The Busan Fireworks Festival takes place every year on Gwangallii Beach, Busan in October. The festival receives over 1 million visitors each year. It is open to all ages and is free unless you decide to pay for paid seating. it was a really great event and I recommend you check it out if you are in Korea.
It was a super packed, event. My friends and I had heard about this ahead of time so we arrived early. We arrived at 1-pm so that we could get good seats for the show even though it did not start until 7-pm. When we got there at 1-pm it was already getting full. Many people tend to go to the Hwangnyeongsan Mountain which is nearby to watch and apparently it is a great view from there.
If you are in the area around October definitely check out the Busan Fireworks Festival.