The Isis festival was a major celebration in Rome in the 3rd and 4th centuries, heralding the arrival of the ship of Isis (navigium Isidis) from Alexandria on 5 March. Besides Isis and Horus, other members of the Egyptian pantheon appear--Serapis, Anubis, Harpocrates, and Nilus. Such coins or tokens with imperial busts were first struck by Diocletian at Rome to mark the arrival of the ship, and the tradition continued through the 4th century; the latest imperial bust to appear is that of Valentinian II. Alföldi proposes that in the Middle Ages the festival associated with the Isis ship (also known as carrus navalis) became the car naval or carnival.
Anonymous issue for the Navigium Isidi, Reduced Nummus, Private mint in Rome, last quarter of 4th Century, AE, (g 1,46, mm 14). ISIS FARIA, draped bust of Isis r., wearing crown surmonted by lotus flower, Rv. VOTA PVBLICA, Isis standing r. on galley. RIC -; C 21 (Elena); Vagi -. Extremely rare