Each one is a real request against a real jurisdiction. The dates below are generated from the same corpus the API serves.
A date is verified, corroborated or compiled. Your code can refuse to act on anything below your own threshold.
A moon-sighting holiday is not a fact until the decree. We return it as estimate with the announcement still pending, rather than a confident wrong date.
Dates are computed from the rule that governs them, so the answer holds for years we have never published — and changes when the rule changes, not when someone remembers to update a list.