Roman legend held that Romulus, after killing his twin brother Remus in a dispute over where to build their city, plowed a sacred furrow around the Palatine Hill to mark Rome's boundary. Ancient scholars, notably Marcus Terentius Varro, calculated this founding to April 21, 753 BC, a date Romans celebrated annually as the Parilia festival.
The story is myth wrapped around a kernel of archaeology: excavations on the Palatine Hill have indeed found traces of an 8th-century BC settlement, lending some credibility to the traditional date even if the fratricidal twins are fiction. Romans themselves treated the founding as sacred history, and the antiquarian Varro, working in the 1st century BC, used lists of kings and consuls to back-calculate the exact year and day.
Every April 21st, Romans celebrated the Parilia, an ancient pastoral festival that predated the city itself, honoring the goddess Pales and involving bonfires, sheep-purification rituals, and leaping over flames. The date's association with Rome's birthday transformed a shepherd's festival into a civic celebration, and it remains marked in Rome today as the city's official birthday, complete with parades of people dressed as legionaries and vestal virgins.