Tokyo says it's puzzled over why Beijing approved national remembrance days to commemorate the Nanjing Massacre and its defeat in World War II, after decades of Japanese pacifism.
The move is the latest in a bitter diplomatic spat between Asia's two largest economies, who are at loggerheads over disputed territory and differing interpretations of their shared history.
State media in China reported on Thursday that the National People's Congress has designated September 3 as victory day and December 13 as a day to remember those killed when imperial troops raped and pillaged the then-capital of Nanjing.
Japan invaded China in the 1930s and the two countries fought a full-scale war from 1937 to 1945.
China says more than 300,000 people were slaughtered by Japanese troops in a six-week killing spree in Nanjing, which started on December 13, 1937. Some foreign academics put the figure lower.
It was unclear what significance the formal "national days" will have, although they are not expected to be public holidays.
Japan's top government spokesman Yoshihide Suga said Friday he could not understand why China had made this change at this point.
"I can't deny there is a question why they have to set up these commemoration days more than 60 years after the war," he said.
"But this is a domestic matter for China, so the government declines to comment on it.
"Japan's position on World War II has not changed a bit, and Japan has followed the path of peaceful nationhood since the end of the war, which has been highly commended by the international community," he added.
Tokyo and Beijing are embroiled in a series of rows, including a long-running diplomatic set-to over disputed islands in the East China Sea.
Tensions rose further late last year when Japanese Prime Minister Shinzo Abe visited the Yasukuni shrine, which honours Japan's war dead, including convicted war criminals.
