I thought tenebrae was held on Good Friday, symbolizing the darkness after the crucifixion. I am curious why your church celebrated it on Palm Sunday?
In our old church, we always held the Tenebrae service on the evening of Maundy Thursday, as a preparation for Good Friday.
In the pre-VII days, Tenebrae was normally done from Spy Wednesday through Good Friday. At our church, Holy Trinity in Boston, the TLM community has been doing the whole thing on Wednesday evenings. This year's appears to be our last, since the archbishop has slated our church for closure in the near future, and the Latin Mass folks are to leave on April 15. Our Tenebrae service is quite beautiful and starts at 7 PM for anyone in the Boston area who might be interested.
Also, the article states that tenebrae is the Latin word for "shadows." It is better translated as "darkness."
Don't know anything about the tradition and this is not my parish. Ultimately, the purpose of this is to prepare us for Holy Week. May you and yours have a blessed one.