This is an ancient Dionysian custom, brought in Greece by the people who fled the Black Sea region of Turkey between 1922 - 1924, following the Lausanne treaty.
The dates are not standard but Momoyer might be seen in different days, between December 18 to January 8. In Athens, this usually happens around December 20, as the participants might be visiting from Northern Greece and may want to participate in the ones taking place in their village.
I took part and made images in the event of Momoyer in two different local clubs of Kallithea. In this one, all the participants are locals and come from this club.
The Momoyers are full of jokes and tricks around the streets. They start at the location given here, which is the office of the local Pontic club organising the event and go around the streets of Kallithea teasing people but also offering Tsipouro (traditional alcoholic drink made by distilled grapes).
In this specific one, one man carries his dead brother on his shoulder, which is a puppet. This takes part in the theatricals.
Apart from the Momoyer, who are the dancers, dressed in colourful costumes, there are many Satyr characters, such as the bride, which is actually a man, the old man and woman, the devil and the police officer.
In each place they visit, they will perform a ritual dance, which is characteristic only to to Momoyer event. Pontic people will not dance this one in any other event. Sometimes, the bride may be stolen by a spectator and the Momoyer will come to the rescue. They will catch the culprit and take him to the police officer to arrest him.