Gyeongbokgung Palace opens its gates at night from September 2 through October 2. Hours run from 7 p.m. to 9:30 p.m. (last entry 8:30 p.m.), closed only on Mondays and Tuesdays.
Admission is 3,000 won. Online reservations open for up to 3,000 people a day, with up to four tickets per person, available until the day before the visit: same-day online booking isn't possible.
The real question is what happens if you show up without a reservation. For exactly that reason, 300 same-day tickets are set aside every day for on-site sale to foreign visitors. With a passport or other ID, you can buy up to two tickets on the spot. Supply is limited, though, so it pays to arrive early.
There's also a way in for free, based on age or dress. Foreign visitors aged 6 or under or 65 and over, along with anyone wearing hanbok, can enter without a reservation. ID is still required for the age exemption, and hanbok must meet Gyeongbokgung's free-entry guidelines. A guardian accompanying a child still needs their own ticket.
This season's night opening covers Gwanghwamun, Heungnyemun, Geunjeongjeon, Gyeonghoeru, Sajeongjeon, Gangnyeongjeon, Gyotaejeon, and Amisan. The event runs rain or shine, so bring an umbrella, and it's worth checking the official notice again before you head out.