The Corpus Christi celebration, called Boze Cialo in Polish, is a very big feast in Poland and it always was, at least since I can remember. It always takes place on one of the Thursdays (depending on the date of Easter) in June.
This was one of few religious days that was also observed by the communist state, because it was also a day off from work. The Communist government wanted to ignore it as much as possible ... the best American movies were played on TV during the processions. Still, the majority of people preferred going to the outdoor processions rather than staying home to watch movies.
Every year there were continuous tensions between the church, which wanted to receive permission to have as many outdoor processions as possible, and the government, which blocked the processions as much as it could. Still, almost every year many outside altars were built by parishoners and volunteers, and people demonstrated their religious beliefs by hanging holy pictures and ornaments from their windows.
Corpus Christi had both a religious and folkloric atmosphere. This was probably one of the few times during the year when people, mainly from rural areas, wore their traditional regional dress, making it an excellent time for tourists to see Polish folklore at its best.