Oyama's Dog Festival (oyama inu matsuri 大山犬祭り) is an old tradition from the rice-farming village of Oyama that was first celebrated over 300 years ago.
During that day, the villagers of Oyama organize grandiose parades and rituals in honor of Mekke-inu, a heroic dog that, according to a legend, saved the village from demons. The festival starts inside Oyama village and ends at the Sugio (or Suginō) shrine 椙尾神社.

The legend of Mekke-inu:

A long time ago, two demons were occupying Sugio shrine. The villagers tried everything possible to get rid of them. But everybody failed. Every year on the 5th of June, the demons demanded to get the youngest and most beautiful girl sacrificed to them. If the villagers refused, the demons destroyed the fields and cast a spell to condemn the villagers to a whole year of famine.
The victim was chosen by the demons themselves, who shot a white feathered arrow into the door of the house where the most beautiful little girl lived.
One day, a Shugendo pilgrim who was heading to Dewa Sanzan stopped at Oyama by chance. He saw Oyama's villagers organizing a majestic parade, but all were looking sad. Why would the people look so unhappy when such a fantastic festival was ongoing? He asked a villager what it was all about.
The villager told him every year, their most beloved girl was destined to die between the hands of the demons.
The pilgrim couldn't believe him, so he decided to secretly follow the carriage where the little girl was locked in. The pilgrim hid behind a rock in the Sugio shrine's forest. After the villagers left the carriage alone, two demons looking like giant old men (onyudo 大入道) showed up, drooling over the sacrificial carriage. They took the little girl out of her cage while singing cheerfully: "May Tanba's Mekke-inu never hear about this". They tore the little girl in half and left the shrine, happily talking about doing the same thing the following year.
The pilgrim was horrified, but he knew there was someone who could stop the demons, thanks to their macabre song.
He left for the lands of Tanba (current Kyoto), looking for the said Mekke-inu. When he found him, he realized it was nothing more but a regular dog.
Without expecting much, the pilgrim went back to Oyama village accompanied by the dog. It took them several months to go back to the North. When they arrived, it was the day of the festival. Because there was no much time left to think, the pilgrim had to find something fast. He decided to trick the demons by putting Mekke-inu inside the carriage instead of the young girl.
The villagers carried the vehicle as usual and left it in front of the Sugio shrine. Just like always, the two old demons appeared, rejoicing about what they were going to do to their young victim. But, as they opened the door, it was not the young girl they saw, but Mekke-inu the dog, ferociously jumping on them.
After a long fight, the two demons were finally defeated, lying in a pool of blood. But next to them, Mekke-inu, too, was lying lifeless.
The pilgrim cried the loss of his friend. But he was not the only one saddened by Mekke-inu's death. The villagers were moved by the dog's courage, who sacrificed himself to save them. They decided to hold a festival in honor of Mekke-inu the dog, so the next generations never forget the dog's courage.
Nowadays, the festival celebrates the courage and the loyalty of all dogs.