I planted early. Everything died.

My husband said the warm days meant spring. He is no longer in charge of the seed. — Farm wife, about 3500 BC

Egypt's farmers planted by the Nile flood, and when a certain star came up just before dawn, they knew the water was on its way. The official calendar had 365 days, which is close but not close enough. It ran a quarter day short, so its flood season drifted off and didn't line up with the real one again for 1,460 years. In 238 BC the priests ordered an extra day every four years. Then, by the look of it, they never got around to it. Committees haven't changed much.

Noted: the extra day those priests skipped is the same fix your calendar uses every four years.

The full file

The leap day finally stuck in Rome, and Egypt supplied the arithmetic. When Julius Caesar reformed Rome's calendar, the astronomer who did the figuring was Sosigenes of Alexandria. To catch the calendar up with the seasons, Caesar made 46 BC last 445 days, then put an extra day in February every fourth year.

The priests' order from 238 BC is known from stone slabs, written three ways: hieroglyphs, demotic and Greek. The first was found in 1866 at Tanis in the Nile Delta. It stands over seven feet tall and is in the Egyptian Museum in Cairo.

They're still turning up. In September 2025 Egypt announced a new, complete copy, a four-foot sandstone slab from the eastern Nile Delta, written only in hieroglyphs. They carved that order in stone at least seven times. By the look of it, nobody followed it.