Paschal refers to the passage of God's destroying angel on the night of Passover. The angel "passed over" the houses of the Israelites but killed the firstborn child in the houses of the Egyptians. A sacred mystery is a divine mystery which cannot be grasped by mere human reasoning and can only be revealed by God through grace.

Paschal Triduum From Wikipedia, the free encyclopedia Christ Washing the Feet of the Apostles by Meister des Hausbuches, 1475 ( Gemäldegalerie, Berlin) Paschal Triduum and other named days and day ranges around Lent and Easter in Western Christianity, with the fasting days of Lent numbered Liturgical seasons Pre-Christmas Advent ( Western)

The entire Paschal Cycle refers to all the observations dependent on the date of Pascha. It therefore starts at Zacchaeus Sunday. If it's to be renamed "Triodion" would be better since the Pentecostarion, aka "the Flowery Triodion", is technically volume 2 of the whole Triodion, the Lenten Triodion being volume 1. But I think it's fine where it is.

The Paschal cycle, in Eastern Orthodox Christianity, is the cycle of the moveable feasts built around Pascha (Easter). [lower-alpha 1] The cycle consists of approximately ten weeks before and seven weeks after Pascha. The ten weeks before Pascha are known as the period of the Triodion (referring to the liturgical book that contains the services for this liturgical season).