Keirin.jp and google translate are getting me closer
Where in JP? The races are kinda random all over the country, but
開催日程｜KEIRIN.JP includes all proper races, not including trials races.
https://keirin-station.com/keirindb/list/calendar/ has everything broken down by month.
There may be a “Track Party” some time in spring (though details have not yet been posted) in Shizuoka. Its like a big velodrome event with various vendors, music, and different kinds of track racing. The website is pretty out of date, still lists the Fall 2018 event, but I know they did it last year as well.
I went to an event in Kyoto in October, and had a fantastic time. The velodrome was super dilapidated and there weren’t many people there, but that added to the charm of the whole experience.