The Decree of Canopus is a trilingual inscription in three scripts, which dates from the Ptolemaic period of Ancient Egypt. It was written in three writing systems: Egyptian hieroglyphs, demotic, and Greek, on several ancient Egyptian memorial stones, or steles. The inscription is a record of a great assembly of priests held at Canopus, Egypt, in 238 BCE. Their decree honoured Pharaoh Ptolemy III Euergetes; Queen Berenice, his wife; and Princess Berenice.
In 1866, Karl Richard Lepsius discovered at Tanis the first copy of this Decree (this copy was originally known as the 'Şân Stele'). Another copy was found in 1881 by Gaston Maspero at Kom el-Hisn in the western Nile Delta. Later on, some other fragmentary copies were found. In March 2004, while excavating at Bubastis, the German-Egyptian 'Tell Basta Project' archaeologists discovered yet another well preserved copy of the Decree.
This is the second earliest of the series of trilingual inscriptions of the "Rosetta Stone Series", also known as Ptolemaic Decrees. There are four such Decrees, the earliest is the Decree of Alexandria from 243 BCE, the third being the Decree of Memphis, for Ptolemy IV in 218 BCE, and the fourth, final decree, being the Memphis Decree Rosetta Stone, inscribed for Ptolemy V, in 196 BCE. Having a greater number of different hieroglyphs than the Rosetta Stone, the Canopus Stone has proved crucial in deciphering them.
The inscription touches on subjects such as military campaigns, famine relief, Egyptian religion and governmental organization in Ptolemaic Egypt. It mentions the king's donations to the temples, his support for the Apis and Mnevis cults, which enjoyed huge success in the Macedonian – Egyptian world, and the return of divine statues which had been carried off by Cambyses. It extols the king's success in quelling insurgencies of native Egyptians, operations referred to as 'keeping the peace.' It reminds the reader that during a year of low inundation, the government had remitted taxes and imported grain from abroad. It inaugurates the most accurate solar calendar known to the ancient world, with 365¼ days per year. It declares the deceased princess Berenike a goddess and creates a cult for her, with women, men, ceremonies, and special 'bread-cakes'. Lastly it orders the decree to be incised in stone or bronze in both hieroglyphs and Greek, and to be publicly displayed in the temples.
The Decree of Canopus attested the existence of the ancient city of Heracleion, which is now submerged, and only recently been excavated. The Decree informed us, in its Greek version, that a synod of priests was held in the city of Heracleion during the reign of King Ptolemy I.
The traditional Egyptian calendar had 365 days: twelve months of thirty days each and an additional five epagomenal days. According to the reform, the 5–day "Opening of the Year" ceremonies would include an additional 6th day every fourth year. The reason given was that the rise of Sothis advances to another day in every 4 years, so that attaching the beginning of the year to the heliacal rising of the star Sirius would keep the calendar synchronized with the seasons.
This Ptolemaic calendar reform failed, but was finally officially implemented in Egypt by Augustus in 26/25 BCE, now called the Alexandrian calendar, with a sixth epagomenal day occurring for the first time on 29 August 22 BCE. Julius Caesar had earlier implemented a 365¼ day year in Rome in 45 BCE as part of the Julian calendar.