Navigium Isidis
The Greco-Roman cult of Isis grew from the thousands year old cult of the Kemetic goddess Aset. Through many centuries of evolution it received its definitive form under the Ptolemies, where it was linked with the cult of Serapis. Isis became the goddess of sailors, and sailors haling from Ptolemy's capitol at Alexandria carried the cult all over the Hellenistic realms. As the Romans devoured the Hellenistic East, their lands in turn were consumed by the militants of the Isiac cult. By the time of Caligula, the cult of Isis which had been once repressed by the conservative Republican Senate seems to have infiltrated every imperial port, and had an especially heavy presence in Rome's port town of Ostia.
In the new Roman solar calendar, March 5th and 6th was a time of celebration by the cult adherents. This was the Navigium Isidis, The Vessel of Isis. March 5th was the start of the sailing year in much of the Mediterranean, and Isis had become a divine patron of sailors, for myth had told how she had searched the waters for remains of her murdered brother-husband Osiris. There was also a powerful metaphor at work: the sea was the waves of fate upon which humanity drifted, and Isis was the savior goddess whose intervention could steer humanity on course.
In his Golden Ass, Apuleius gives us an interesting look at this colorful festival. There was a kind of carnival customed procession of people, many of whom were not neccesarily initiates in the cult but simple participants in the joyous activities. The men arrived first. Some dressed as a certain profession, some as animals, still others as mythological figures. A few cross-dressed as women, perhaps imitating a practice of the Dionysian cults. Then came the women. They were clothed in white, and had flowers in their hair. They flung flower petals to the streets as they passed by.
The truly faithful of the cult followed behind. Many carried lamps - lamps having a connotation of protective magic in Ancient Egypt. Others carried torches and candles. Singers and musicians dedicated to the god Serapis proceeded next, who with flute and pipe played cultic melodies. Next came new initiates to the cult. The women were veiled, the men shaven headed, all were clad in linen and shaking a sistra.
The priests and ministers of the cult were next in procession. They carried holy water to preside over the ceremonies, as well as various items of cultic significance. Some of the priests were dressed as gods connected with the Isaic cult, such as Anubis. Finally, the high priest of the cult brought up the rear.
A newly fashioned boat decked with Egyptian symbols was waiting for the procession in port. There the high priest prayed over the boat and ritually consecrated it with a torch, an egg, and some sulphur. The vessel was filled with spice and other offerings. The ropes were cut, and off the boat drifted into the high seas.
Back at the temples, the high priest and his ministers said prayers for the health of the emperor, the Senatorial and Equestrian orders, and the Roman people (rather generous of a cult once persecuted by the Romans). In Greek the priests blessed sailors and announced the start of the sailing season. The crowd then adorned a silver statue of Isis with flowers and greenery, and kissed her feet.
The cult may have varied from port to port, but in general it was cause for Spring time merriment.