It doesn't look like Tenebrae is scheduled on your bulletin. Don't be alarmed...it's sort of rare to have it.
Tenebrae is the fully chanted version as given in the Liber Usualis of the Offices of Matins and Lauds for the 3 Days of the Triduum. Fully chanted as opposed to simple recitation or private recitation. The sanctuary is set up as follows:
Empty Tabernacle
6 Lighted Candles on the Altar plus a Triangular Candelabrum w/ 15 Candles lit placed near the Communion Rail on the Epistle side.
A Lectern in the center
Choir stalls/seats on either side for the priests and any men (Schola) assisting.
As each psalm (Matins has 9 and Lauds has 5) is completed, one of the 15 Candles is extinguished.
Matins also 9 Lessons, chanted by priests and Schola members.
At the end of Lauds, the Canticle of Zachary (Benedictus) is sung, during which the 6 Altar Candles are extinguished and all the lights in Church are turned off.
Finally, the MC takes the final Candle (number 15) from the Triangle and hides it behind the altar, while all sing the "Christus Factus..." antiphon. Then a period of silence ensues followed by a loud knocking on the pews. The server then returns the candle to the triangle and extinguishes it.
All leave in darkness.
The word "Tenebrae" literally means "Darkness".
OOOH! Thanks for that answer. Is there a link that I can read more about this? Maybe it would be included in a missal?