Yamagata has a famous traditional local food called "Imoni" (芋煮). Imoni is basically a soy sauce based soup with taro (里芋 in Japanese), beef, green onion, etc. I quite like the imoni that my grandma cooks.
As you see the huge pot in the picture above, Yamagata holds Imoni festival in summer where imoni for thousands of people is cooked with the huge pot and served on the riverbank. Because this festival is so popular and thus crowded, local people have come up with their own way of holding a small imoni festival. It is like BBQ, but instead of grill you bring a large pot and cook their own imoni with a group of small number of people like family, friends and/or colleagues.
Imoni goes very well with sake. There will be imoni party organised as part of my tour in summer time!