When you come decides what you pack

Late June through July is the rainy season. Expect sudden downpours rather than all-day drizzle, so keep a folding umbrella in your bag.
July and August are hot and humid. Indoors the air conditioning is fierce — bring one light layer.
September and October are the easiest months to travel. The Chuseok holiday is the exception: trains and coaches sell out weeks ahead.
November to March is dry and cold. January mornings can drop below minus ten, so pack gloves and a hat.
Spring and late winter bring days of heavy fine dust. KF94 masks are sold in every convenience store when it happens.
If you have booked anything outdoors, check the forecast again the morning you set out. Whether a rained-on event goes ahead is the organiser's call.