Olive Young does not publish an annual sale calendar in advance, so there is no fixed 2026 date to hand you. What exists is a pattern: one large store-wide event roughly once a quarter, historically falling near March, June, September and December, plus a rotating set of 1+1 and 2+1 tags on individual products almost every week in between. The exact weeks shift year to year and get announced close to the day.
Three places to check for the current event, in order of how much I trust them: the shelf tag in front of you, the event banner in the Olive Young app, and the signage at the store entrance. If you are already in Korea, shop while you are here. If you are choosing between two trip dates and everything else is equal, the end of a quarter is the better bet. Do not move a flight for it.