The Two Day March

Dewey Lane is sick of being an English teacher in Japan. His beautiful Swedish girlfriend has just dumped him for a Tokyo banker. But he's already paid for a vacation with his ex and her friends-to a famous hot spring resort, deep in the Japanese countryside. Waiting for the girls to arrive, he discovers the town is celebrating their first annual "Two Day March."

The longer Dewey stays, however, the more he learns about the event's sinister nature. Day one is euthanasia day-where the young have at the old, taking out the weak, the senile, and the embittered. But on day two, the elderly take revenge...with their own brand of homicidal traditional justice
From inside the novel...
They paused and sat down. Everyone was exhausted. It was slow going in the dark, and the hike from the car had been mostly uphill. Dewey knew that it would be even steeper after the bridge. He watched as the mayor sat down next to Bibi. Felt a pang of jealousy.
"So you see, Miss Bibi, today we have been killing as many of our elderly as possible. They are not allowed to fight back. They can only hide, or shelter in a field at the top of the mountain. But tomorrow, the tables are turned. They are allowed to kill the adults below the age of sixty. And we are not allowed to fight back. Nor can we use weapons. There is also the fact that we are outnumbered."
"By how many?" asked Bibi.
"Easily four or five to one," said Dewey.
"The only way to survive is to get back to the town of Kita-Yamanouchi. That is our safe area. We begin the march again at dawn. So you see, searching for you is robbing us of precious time."
"Then why don't you have it at night?" asked Bibi.
A perfectly obvious question.
"Too dangerous," said the mayor. "Someone could get injured. Or fall off a cliff."