Dziady is a pre-Christian All Souls ritual during which those who live celebrated their connection with those who passed away. It was belived that souls sometimes come back to places where they were at their lifetime. By this ceremony people were trying to make contact with their ancestors. The purpose of this ritual was to gain favour from deceased relatives who were belived to be keepers of fecundity and good harvest. The word Dziady comes from Belorussian language and was mainly used in Eastern parts of Poland. Although it was called differently depending on the region the ritual itself was practiced commonly by Slavic people as well as by other European nations.
In Slavic tradition Dziady was celebrated few times a year. It differed by the region but usually Dziady was celebrated from 3 to 6 times during the year. In Poland most important ceremonies were in Autumn at night from 31st of October to 1st of November. Nowadays it is commonly known as All Saints Night that precedes All Saints Day on the 2nd of November.
The Dziady ritual was precisely described by Adam Mickiewicz. He used it as the bonding element in his romantic drama under the same title.