The Seoul Lantern Festival is an annual event held along the Cheonggyecheon Stream in central Seoul, typically running for a few weeks in the November–December window, featuring large illuminated lantern displays along the walkable stream corridor. It's free to attend and draws large evening crowds, especially on weekends.
Planning around it
Exact dates are usually confirmed by the city closer to the season, so if the festival is a reason you're timing a trip, check the current year's official announcement rather than assuming last year's dates repeat. Evenings are considerably busier than late afternoon, and the stream's narrow walkway means weekend evening crowds can get genuinely dense — a weekday visit gives a calmer experience of the same lights.