how to add holidays to iCloud calendar on a Windows PC

Hello there rocksea775,

Thanks for reaching out to the Apple Support Communities! It's currently not possible to add holidays to iCloud own a PC. The Holiday calendar is specific to the iOS/macOS Calendar app, so those events don't sync to your iCloud account in the same way as an event that you create. You can find more about this, and how you might be able to find third-party holiday calendars you can subscribe to, here: About holiday calendars on iOS and macOS. Another option you might try is to Use Calendar on iCloud.com.

Outlook can display it's own holidays: Add holidays to your calendar in Outlook for Windows

While it's not possible to toggle on the Holiday calendar on the iCloud website, you can always send your feedback to Apple here: Product Feedback

Cheers!