Holy Saturday or Sabbatum Sanctum is also called ""Silent Saturday"" because the church bells will not toll until Easter. I was unable to find any specific information on church services during Bach''s time, but I suspect that there was probably no music allowed in mass (Tempus Clausum).
However, this is an official day on the liturgic calendar, so I''m eager to fill the slot with some of Bach''s magnificent vocal music. First off the wonderful Motets BWV 225 to BWV 230, here in a great recording by Philippe Herreweghe and the Collegium Vocale Gent.
A lot is not known about these motets, probably written in Leipzig between 1723 and 1728. They are probably written for funerals, so they are appropriate for this Silent Saturday. They are also the only vocal works by Bach that stayed in the repertoire after Bach''s death up until the Bach revival in the 19th century. They were also among the first music by Bach ever to be printed.
Next is the motet Ich lasse dich nicht, BWV 1165, formerly numbered as Anh. 159, so part of the ""Anhang"" section of the BWV catalogue, containing works considered lost, doubtful, and spurious. But thanks to the research by Daniel Melamed and the publication of two articles in 1988 and 1995 it is now accepted as a work by JS Bach, and given a formal BWV number, 1165. It is most likely a very early work and may contain parts composed by his uncle Johann Christoph Bach.
I''ve also added the possibly biggest lost work of Bach: his Markus Passion. The only existing copy of the score (but not an original manuscript) was never published, and was lost in a fire in 1945. But I''ve come to realise the amount of reconstruction work that has taken place over the years (just have a look at the Wikipedia lemma to appreciate the work: https://en.wikipedia.org/wiki/St_Mark_Passion,_BWV_247).
It was already known that this passion is most a parody work (lending music from earlier cantatas like Laß, Fürstin, laß noch einen Strahl, BWV 198. With the discovery of a libretto (but no score) in St-Petersburg of the version Bach performed in 1744 the structure of the passion was clear (at least for the 1744 version). Based on that discovery, Alexander Grychtolik made a reconstruction, of which in 2019 Jordi Savall made a very qualitative recording, which I think is really worthwhile to listen to. It helps us to appreciate the work it may have been, and regret its loss even more. Considering that there is most likely also a Lucas Passion of which no trace remains... I hope this reconstruction gives you some consolation.