Thanks, Dave. A logical explanation for what to the uninformed, might seem an odd usage.
In Spanish it’s Viernes santo—“Holy Friday”.
A few years ago, I was travelling in California (where else?). That year, Easter and Passover coincided pretty exactly. I entered a supermarket --- the delicatessen counter was all dressed up with festive trimmings, and on the counter was a prominent sign: “Have a Happy Good Friday”. I moved further along the counter and there above the hams and sides of bacon was another, equally festive sign: “Have a Happy Passover”. Nothing if not ecumenical...... the only thing missing (it was the wrong time of year, but who cares --- business is business) was Io saturnalia!
(Edited to sharpen point)