All are very valid questions, and this is what we found out.
Who organizes the ceremony?
Because the Ja Matsuri festival is designated as a cultural asset in Tochigi prefecture, you can be certain that the city of Oyama has something to do with organizing the event. And yes, turns out that the main organizer is actually the Oyama city tourist association. That body, in conjunction with Ujiko members are responsible for putting on the show.
“What’s Ujiko?” I hear you ask.
Ujiko is a group of local people who worship the same local god, Ujigami 氏神. Uji (氏) means “clan” in English.
Before taking a dip in the pond
Back in the olden days, Ujigami was considered as the ancestor of local power. Ujiko are its (his?) offspring and relatives of the clan who worship the same Ujigami. So basically Ujigami is a mythical superhero (something like a patron saint, I guess) of the clan, and thanks to him, the clan has an important name, wealth and influence. Or at least, that’s how the story goes.
Every shrine has Ujiko members, and such members in Oyama, along with the tourist office are responsible for organizing the ceremony.
Poster for the festival
The shrine’s part is to engage not only in religious services, but also to include public and social aspects of the event, somewhat similar to what you could expect a community center to do.
And the tourist office job is to energize the local area by getting involved in organizing ritual ceremonies to make a buck. Because, let’s face it, at the end of the day, it’s also about money – the money that people attending such events so willingly spend.
How are people chosen to participate?
Young boys and girls in each district, who are taught by some old dude how to make the snake, are chosen to dip themselves in the filthy pond. I guess it all depends on whether their parents are willing to participate as well. The rest is all about connection. You know someone who’s doing it, and you’re asked to do it too.
When it comes to traditional Japanese music (taiko drumming and such), there are groups that preserve traditional instruments and teach the young ones to play. Again, kids are encouraged to participate and participating carries with it instant respect and prestige.
If this festival is like many others in other cities, then members of public services (like local fire department guys and their families) are also drafted into participating. Sometimes, they do it as “volunteers” and sometimes they get paid. It all depends on how determined any given district is to appear “involved” and prestigious.
Is it an ecumenical event or do only "shintoists" participate?
It’s a circus. Any religious significance is symbolic only. The festival is an excuse to eat, drink and have a good time. And with this particular festival it just so happens that this particular good time involves getting soaking wet in a shrine pond.
And if you're not into getting soaking wet, you can always have some yakisoba or candy apples.
Personally, I'd take the apples:
or a bunny:
Or even a poor turtle, who no doubt thinks "Shit, I better get out of here before the crazies invade my pond":
Part one of our report is here.