subbotnik
A Saturday designated for community volunteer work, such as cleaning the streets, after the October Revolution in Russia.
historical
In any society there are the mavericks and you will find them among the Soviet students too—just as you will find the committed Komsomol activists, who take the attendance register, organise the subbotniki (the cleaning-up of the hostels and yards), run the student council, organise political meetings and write references for job applicants).^([sic])
One who took part in this work.
historical
Though the individual was the initiator, his or her success enlisted others in group movements, such as the Subbotniks, who in 1919 gave every Saturday (Subbota) without pay to work on the railroads, repairing cars and engines and loading freight; […]
A member of a Russian sect of Sabbath keepers / Sabbatarians.
The Subbotniki (Sabbatarians), for example, adhere to many Jewish observances and accept Jesus only as a prophet. In this, they differ from the predominant Voskresniki (Sunday observers).
Where it comes from
Borrowed from Russian суббо́тник (subbótnik), from суббо́та (subbóta, “Saturday”) + -ник (-nik).
Adapted from the Wiktionary entry subbotnik (CC BY-SA 4.0).