Oct 17 to 25This is a festival of arts and culture that does not stay on dry land: part of the autumn Hangang River Festival (한강페스티벌 가을) takes place out on the Han River itself, and the rest in the parks along its banks. Seoul's Future Hangang Headquarters puts it on.
Its two main areas sit a long way apart on the river, so plan on one per outing rather than trying to see both in an afternoon.
Before you set out, look at the official festival page for what is on at each spot, and for whether anything on the water asks you to reserve a place.