Kept: 27 September
On this day in 1822, Jean-Francois Champollion read out a letter to the Academie des Inscriptions in Paris. In it he showed that he had worked out how to read Egyptian hieroglyphs. The letter is dated 27 September 1822.
The stone that made it possible is the Rosetta Stone. One decree, cut in 196 BC, written three times: hieroglyphs, demotic, and Greek. For two thousand years it sat there with the answer on it, under two other languages, and nobody got through.
What Champollion had that the others didn't was Coptic, a late form of Egyptian still used in church. He could hear the old sounds inside a living language. That was the seam.
One true thing: the answer was never hidden. It was written down, three times, and waiting for whoever could read the third language.