Feb. 22 is one of the annual big days of diplomatic tension between South Korea and Japan as officials in the Japanese city of Matsue commemorate what they call "Takeshima Day," a moment to claim ownership of small islets, known as Dokdo in South Korea, that are between the two countries. South Korea has controlled them since the 1950s.

In Seoul on Friday, a small group of South Koreans staged a protest of the event in Matsue in front of the Japanese embassy. And the South Korean foreign ministry later in the day...