Anonymous wrote:...Yom Kippur is only every major and even minor calendar you can consult on when to pick a birthday party date.
I wouldn't attend. Even if I weren't religious, I grew up where if you weren't going to synagogue on Yom, you didn't do anything. I'm pregnant and not fasting, I plan on attending for an hour or two, but won't spend the rest of the day running errands. It's an important day.
I'd see if your daughter can have a smaller get together with the birthday girl at a later date to celebrate since they're such good friends - this will be a good lesson to her on the importance on the holiday. Just an idea.
It's not on the calendar that's hanging in my kitchen, nor is it showing up on my Google calendar, so to me it seems possible that they really didn't know.
If I were this parent, I'd be bummed when I realized I had scheduled it for Yom Kippur and I'd try to watch out for that in future years (and I'd hope someone would mention it to me, politely, so that I would know).