Why is this Day of the Lord referred to as "Passion Sunday" and how does it serve as the gateway into Passiontide?

With Passiontide we approach the conclusion of our Lenten journey, and so we intensify our focus and perhaps our emotion, shifting from the human Jesus to the glory of Christ

The Gospels now focus on the Passion of Our Lord. “Passion” in the Christian sense, does not just mean an intense emotion; the Passion describes what happened to Jesus at the end of his life, at a point when He stopped being a doer (preacher, teacher, healer, worker of signs and miracles), and became the man to whom things were done. That is, when Jesus became the object of other people’s actions, rather than acting himself. The Passion therefore encompasses suffering, endurance, refusal to retaliate and also love – but expressed in human, physical form and of great intensity. In Catholic churches, one of the most striking changes in the Passiontide is that the crosses and statues are draped, as an outward sign of the Church’s inward sorrow. It is not difficult to understand why the wailing garments are placed over the statues, which could distract us from the meditations of the Passion.

Jesus “the doer” is, perhaps more straightforward for us to understand. We can read and picture in our minds the healings, the conversations and travels of Jesus. But in the Passion of Our Lord, the time at the very end of His human ministry when He became the one who suffered, we now need to worship with the heart, for we are now contemplating not just what Jesus did that saves us, but what he was, and is and ever shall be.