Navigium Isidis
The Navigium Isidis or Isidis Navigium was an annual ancient Roman religious festival in honor of the goddess Isis, held on March 5. The festival outlived Persecution of Pagans by the [Christian Roman Empire|Christian persecution] by Theodosius I|Theodosius] and Christian persecution of paganism after [Theodosius I until the fall of the Roman Empire#Arcadius laws, 395|Arcadius' persecution] against the Roman religion.
In the Roman Empire, it was still celebrated in Italy at least until the year 416. In Egypt, it was suppressed by Christian authorities in the 6th century.
The Navigium Isidis celebrated Isis' influence over the sea and served as a prayer for the safety of seafarers and, eventually, of the Roman people and their leaders. It consisted of an elaborate procession, including Isiac priests and devotees with a wide variety of costumes and sacred emblems, carrying a model ship from the local Isis temple to the sea or to a nearby river.
Modern carnival resembles the festival of the Navigium Isidis, and some scholars argue that they share the same origin. Many elements of Carnival were in turn appropriated in the Feast of [Corpus Christi|Corpus Christi festival], most prominently in the Iberian Peninsula.