The Ten Missing Days of October 1582
The Julian calendar had been running about eleven minutes fast per year since Caesar, which sounds harmless until you let it compound for sixteen centuries and Easter starts drifting toward summer. The fix was to delete the accumulated error outright: in the countries that adopted it immediately, Thursday 4 October 1582 was followed by Friday 15 October 1582. Ten days simply did not occur.
Why it was classified
Not classified — announced by papal bull, which was the sixteenth century's version of a global software patch note. The difficulty was not secrecy but rollout: adoption depended entirely on whether a given ruler took instructions from Rome.
Why it matters
Because the patch shipped over 340 years, the size of the gap kept growing. Britain and its colonies dropped eleven days in 1752, which is why George Washington has two birthdays. Russia dropped thirteen in 1918, which is why the October Revolution is commemorated in November, and Greece did the same in 1923. Somewhere in the historical record is a stretch of dates on which, locally, nothing happened at all.