Q. Were J.S. Bach's "Passions" (St. Matthew Passion, St. John Passion) written to be performed on Good Friday?

Yes. The St. John Passion was first performed on Good Friday, 7 April 1724.  The St. Matthew Passion was first performed on Good Friday, 11 April 1727 (the revised version was first performed on Good Friday, 30 March 1736). [Source: Grove Music Online, s.v. "Bach, Johann Sebastian, works list, BWV 244 and 244b (St. Matthew Passion) and BWV 245 (St. John Passion).]