We have not held any CanJams in Japan, and the closest CanJams would be Shanghai, Shenzhen, and Singapore. Since you’re looking for events in the Japan I recommend checking out the shows put on by e-earphone or Fujiya-Avic. Between them there are 4 big gatherings a year and a number of smaller regional shows. I was lucky to attend a regional show in Nagoya last year and it had a range of gear to try.
Japan has a flourishing headphone scene, and if you know where to look everyday is like a CanJam! Two of the main players are as mentioned Fujiya Avic which is one stop north of Shinjuku station on the 3rd floor of Nakano Broadway shopping mall, and e-earphone shop. I spent a good chunk of a day trying basically all their IEMs on display which numbered around 150, as well as a dozen higher caliber ones behind the counter by request. They also have full size headphones, DAPs, amps, etc. As for e-earphones, that is really the place to behold. With about 5 locations spread throughout Japan, most notably the several in Tokyo and two in Osaka, they boast 100's of IEMs on display for audition and 100's more headphones both summit and mid-fi alike, along with English speaking staff and even an onsite technician for all your cable / etc. customization requests. And the best part is that with their Japanese hospitality and patience, you're basically free to spend all day trying things out with no pressure to buy. But of course you can't resist everything on-hand. e-earphone shop is much more than earphones and they have a website too showing store locations and online deals.
I also recommend the huge electronics store chains Yodobashi and Bic Camera. Though their selection is a bit redundant to the above and staff not as into the personal hi-fi audio scene, they can be an interesting alternative. And as mentioned above there are local gatherings with some. Though I can't speak for how the current outbreak is affecting those, and for me personally I would bring along alcohol wipes before trying any earphones in the current climate, although they do have people to regularly clean the eartips as well.
Osaka is a very special place for the hobby in Japan. Within a few blocks on the same road near one of the e-earphone retail locations (which is quite chill and with diverse gear on display), there are also several historical seeming shops offering top Japanese tube stereo amps, speakers, SS speaker amps, tubes, etc.
If you're in greater Tokyo and you're into vintage and 2-channel audio there were also two or three audio dealers on the side streets in Akihabara. If memory serves, two of them were within a block of each other. Lots of vintage audio gear and some speakers available to try. For headphones, E-earphone updated their Akihabara store from one to two stories, and I don't know if they moved their dedicated custom IEM shop into the new remodel, but if not that store was just around the corner. Fair warning like Levi mentioned, the Japanese headphone shops can drain both your time and wallet with ease.
Make sure to check out the Headphone Festival and Potafes websites for the schedule and updates. If you're near Tokyo you'll have access to attend more shows than other regions.