When the chunk of people who cannot come all belong to a particular minority group... yes, that's an issue. Likewise you choose to hold a con in a space with no elevators and "only" five people can't come because those five are wheelchair users... that's an issue.
Shavuot is honestly different because the majority of Jews would attend an event on it. It's frustrating when it overlaps a con but it's not like Rosh Hashanah. I am basically asking for three (3) days to black out on the calendar.
For a band, it's is possible to extend your tour by two days. But even if you didn't, someone who really wanted to come could travel to a different city, so again, not as much of an issue. Also... we're not idiots and we understand the concept of a tour. It's different from a single standalone event.
If you couldn't get a venue literally ANY other day of the year... well, idk, start planning earlier next time? Or choose a different venue? Somehow I don't tend to hear about how a con just had to be on Christmas because that's the only time the venue had space.
"Why should my schedule change because of a religion I'm not in? How is that fair to me?" This is literally what Jews do 365 days a year. Our schedules, calendars, speech patterns, language, orientation towards time, hair textures, etc ALL have to be shifted based on the desires of Christian society.
When I was a kid, there was an entire stretch of highway you couldn't use for the month of December because it went by the mall and the Christmas traffic made it standstill every day. That doesn't count as my schedule changing because of a religion I'm not in? My school breaks were timed because of a religion I was not in. Hell, even what day the weekend is is based on Christianity, a religion I am not in.
What if I want to go to a restaurant on Christmas? What if I want to go buy stuff at a store, or go to the zoo, or a museum, or see a play? Do something other than sit around the house? Why should my schedule change because of a religion I'm not in?
The answer is because I don't want the employees of those businesses to have to work on Christmas. If that means it's a day when I'm just chilling around the house, that's fine. Because I actually give a shit about the experiences other people are having.
Anyway, your argument is "it's your fault for being too small a minority group, so you have to suck it up. I should never have to suck it up because I would find that inconvenient."
Aside from the fact that yes, sometimes it can and should be the majority cultural group suffering inconvenience and not just the minority ones every single time, the REASON we're such a small group is because we've had regular massacres and genocides throughout our history that keep reducing our numbers.
The OTHER reason is that unlike Christianity, we don't go around coercing people to join us, so we don't have billions of followers like Christianity does. So idk. Maybe if you dislike Christianity as much as you say you do, you should examine why Christianity has so many adherents that it's able to enforce cultural hegemony, rather than defending that hegemony.
Anyway, the whole point here was that there is a population of people whose calendar aligns with the Christian one despite being atheist and not Christian, and such individuals are culturally Christian atheists. Honestly, this is just observable fact, regardless of your opinion on what anyone should do.
That is to say, regardless of whether it is good or bad or neutral to schedule your local anime con in Rosh Hashanah, the person who did that is likely a cultural Christian, and they did it because either it did not occur to them or, like you, they had other concerns and did not want to change their event based on a religion they're not in. So. They used. Their OWN culture's calendar. The calendar of the culture they are part of. The Christian calendar. Because they're culturally Christian. What is not clicking?