I downloaded the iCal Christian and Jewish calendars. However they overpopulated my calendar with Catholic and Jewish Holidays I’ve never heard of and have no interest in. I think the best answer is to manually add the Christian holidays that you want (U.S. Holidays is absurdly overpopulated too, I have it disabled).
The workaround for me is to manually add Thanksgiving, Christmas, Easter, Passover, Pentecost, tabernacles, etc. each year.
I won’t forget the 4th of July. Memorial Day just sort of happens. I don’t observe Labor Day.
I know I’ll have to check on a few every year but to do that I just enable the Christian, U.S., and Jewish calendars and check the dates for the year. It only takes a few minutes because there aren’t that many I’m interested in. Minor holidays like Presidents Day etc come and go. The mail don’t run and the bank isn’t open... big deal.
Maybe someone will make a Calendar APP where you can delete the holidays you don’t care about and keep the ones you do. How hard could that be? They are not going to force me to care about things I don’t care about. If you’re Catholic there are several to choose from. If you’re Protestant there is nothing. If you want the Biblical Jewish festivals only, you’re out of luck.
Manually is your friend until they make iCal tweakable for your personal use instead of trying to use its to force diversity on you and me. Diversity seems to be the reason for this mess. I’m fine with diversity as long as you do what you want and let me do what I want.