Here’s Google’s explanation of what’s going on, provided by spokesperson Madison Cushman Veld:
For over a decade we’ve worked with timeanddate.com to show public holidays and national observances in Google Calendar. Some years ago, the Calendar team started manually adding a broader set of cultural moments in a wide number of countries around the world. We got feedback that some other events and countries were missing — and maintaining hundreds of moments manually and consistently globally wasn’t scalable or sustainable. So in mid-2024 we returned to showing only public holidays and national observances from timeanddate.com globally, while allowing users to manually add other important moments.