"Mystery Babylon" and "The Great Whore" in Revelation 17 is the Roman Catholic Church. It is a very evil "church"!
About the Sunday thing though, the Apostle Paul himself preached on the first day of the week:
Acts 20:6-7 (KJV): (6)And we sailed away from Philippi after the days of unleavened bread, and came unto them to Troas in five days; where we abode seven days. (7)And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.