add new holiday calendar grayed out

I've experienced the same, and noticed that when I enable the iCloud calendar, then the option becomes available. Basically, I just went to "Calendar->Settings->Accounts", selected the iCloud account and checked "Enable this account". Afterwards, I was able to add holiday calendars, as the menu option is now available. Once I had added the calendars, I could disable the iCloud account again, and so far it looks like the additional holiday calendars are staying.

Hey there davidpeng2023,

We'd like to help by learning more about what's happening, along with ruling out some basics:

1. Is this the first time you tried using this option, or has it worked in the past? If it did work in the past, let us know when it started to become greyed-out.
2. Any issues with seeing the holiday events for your current region within Calendar?
3. Sometimes unexpected behavior can be resolved with a restart. Check out the section ‘Restart your Mac' here: Shut down or restart your Mac

Cheers!