Back in the USSR those volunteers were told to come on their day off to their offices and do some kind of updaid work that wasn’t connected with their usual duties. They had to wash windows, clean yards, plant trees or flowers. This work was compulsory for everybody in different fields: engineers, teachers, doctors, schoolchildren, students and so on. They were usually held on Saturday (Subbota).
Nowadays subbotniks are held sometimes but they are not compulsory and aren’t connected with the office where a person works. In April or May people clean the yards and plant trees and flowers in their neighborhood.