"I just noticed that unlike many desktop calendars (such as the PandoCalendar I had on my old iMac), iCal has no library of holidays at all (much less holidays listed and installable by country or whatever). Any thoughts?"

Launch iCal. In the Menu Bar/Calendar, click on "Find Subscriptions"

Or, is that not what you seek?

Harv
27" i7 iMac (10.13.6), iPhone Xs Max (12.1)

Those who can make you believe absurdities can make you commit atrocities. ~Voltaire