I want to know the answer to the following question:
Which group held secret Milleneum of Christianity celebrations?
All the background I'm provided with is that "Between May and June 1988, Ukrainian Catholics in western Ukraine celebrated the Millennium of Christianity in Kievan Rus' in secret by holding services in the forests of Buniv, Kalush, Hoshiv, and Zarvanytsia.". Is that enough to answer the question?