When to use that day in Korean
Korean: 그날[ geu-nal ]

When to use it
That day refers to a specific day that both people know about. You use it to talk about something that happened or will happen on that day. In Korean, 그날 is common in daily conversation when telling a story or remembering a special day.
Example sentences
그날 우리는 집에서 같이 저녁을 먹었어요.
That day, we ate dinner together at home.
그날 학교에 늦게 갔어요.
That day, I went to school late.
그날 밖에 비가 많이 왔어요.
That day, it rained a lot outside.
그날 엄마가 이렇게 말했어요.
That day, Mom said this.
그날 마트에서 우유를 샀어요.
That day, I bought milk at the mart.
그날 부산으로 여행을 갔어요.
That day, I traveled to Busan.