"During the dangerous 40 minutes, you had better stay home," National Public Safety Commission Chairman Bunmei Ibuki said, warning of possible danger if Mir's final thrust failed.
And Japan's southernmost islands will be the last, populated area along the defunct station's flight path before it plunges into the water.
On Friday, Japanese residents were warned to stay indoors during the 40-minute period when debris from Mir was most likely to fall, although officials admit that the chances of an accident are very unlikely.
"During the dangerous 40 minutes, you had better stay home," National Public Safety Commission Chairman Bunmei Ibuki said, warning of possible danger if Mir's final thrust failed.
"There are two dangers ... One is Mir might leave its orbit and fall, another is its braking thrust might fail," he said.
However, he said that the probability of space station debris falling on Japan was "one in 100 million".
Shipping warning
Pacific islands were also watchful, while New Zealand has issued international warnings to ships and aircraft travelling in the South Pacific area.
It is estimated that as many as 1,500 fragments - some weighing several tonnes - could rain down for 10 minutes.
But the chances of debris hitting ships would be pretty small, said Tony Martin, deputy director of New Zealand's Maritime Safety Authority.
Japan's science and technology ministry said that it would provide a better idea of when the critical period would be as the time approached.
"Four hours prior to the fall, the time can be forecast with about 45 minutes of error... and the area can be narrowed to about 2%," the ministry said.
Russian scientists will be taking pride in providing a precise landing area, but the fall of the "old goat" - as it has been dubbed by cosmonauts - has been mourned among Russians who regard it as one of the few remaining relics of Cold War Soviet achievement.
Destruction simulation images by Analytical Graphics, Inc