SEOUL, South Korea — South Korean President Yoon Suk Yeol’s party vowed Thursday to oppose efforts to impeach him over his failed attempt to impose martial law in the East Asian democracy, as opposition lawmakers said they would vote on the motion on Saturday.

The announcement of the timing of the vote came as Yoon accepted the resignation of his defense minister, Kim Yong-hyun, who played a key role in the chaotic six-hour period of martial law that began late Tuesday local time and…