I just saw a discussion among corporate event planners where one person was upset that event organizers don’t give proper consideration to scheduling over top of Rosh Hashana and Yom Kippur.
I can appreciate the annoyance, when I was still a practicing Christian I would never think to schedule a work thing over Easter or Christmas. We should treat others with consideration, and should be mindful of what others view as important days. But I also don’t know what each religion considers to be major, non negotiable holidays. Do you?
Another question, does it matter where the event is? (for example, in the US should less consideration be given to holidays of religions that have fewer adherents?)
Many cities have some type of religious council. It may be helpful to reach out, as they may have some sort of list maintained. Alternatively, maybe contact the main school district. The superintendent’s office should have such a list for their scheduling purposes.
In either case, these lists should be pre-negotiated. The local religious leaders have already determined which key holidays should be recognized.