“that thread dates back to last year.”
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https://discussions.apple.com/search.jspa?q=easter&after=day
And if you actually bother *reading* the thread you linked, you’ll see that the solution is “resubscribe to US Holidays”. This is a user error issue, not an Apple omission; as an IT support consultant, I see this often around various holidays. As one example, last November I had a user who complained that Apple had deleted Thanksgiving and Christmas and went on a long rant about how this was a case of Apple attempting to delete traditional American holidays. After he finally wound down, I took his phone, turned US Holidays back on, and sent him away.