This is a movable feast so I place it at the head of the month.
While most of us do not understand the hypostatic union, it is easy to see Jesus as divine, sitting in majesty on his throne, yet we can still have trouble trying to get our heads around Jesus' human nature.
This feast celebrates that aspect of him; certainly we can be confused by the Incarnation and our celebration of that at Xmas, but this is not about God becoming human but the human Jesus who was born, grew, laughed, cried, ate, drank, talked, joked, corrected, taught.
When we use the word "sacred" we are talking about something that has been set aside for God, that is out of the mundane. Jesus' human heart, what we think of idyllically as the seat of the emotions, was given over to God's will. We hear of his compassion, his sorrow, his joy - all of these were felt "in his heart" just as for each of us.
We also remember that the spear pierced his human heart on the Cross, that final physical wound symbolizing all of the emotional wounds of that day and the previous evening; the wound he still carries, on his heavenly throne of majesty.
Sacred Heart of Jesus, remind us to turn our heart to you! Sacred Heart of Jesus remember your compassion and have mercy on us!
Debating the hypostatic union is really not for everyone, but what we all can agree on is that sometimes we forget that Jesus was fully human, head and heart. This feast, pointed to by (in what we do not consider irony) several mystics, reminds us of the fullness of God's plan for salvation. Jesus is God, but he is also one of us. He did not need to become human in order to understand our day-to-day sufferings and joys, but he did in order to show us that it is possible to bend the human will to the divine will without losing anything and gaining everything even amid human suffering. That is simple enough for the simplest of believers. As Bonaventure reminds us, the Church is born from the side of the human Jesus.
Now if you are up for the hypostatic debate...
Therefore, since we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast to our confession. For we do not have a high priest who is unable to sympathize with our weaknesses, but one who has similarly been tested in every way, yet without sin. So let us confidently approach the throne of grace to receive mercy and to find grace for timely help.
-- Hebrews 4:14-16
You who have been redeemed, consider who it is who hangs on the cross for you, whose death gives life to the dead, whose passing is mourned by heaven and earth, while even the hard stones are split. Consider how great he is; consider what he is.
In order that the Church might be formed from the side of Christ as he slept on the cross, in order that the word of scripture might be fulfilled – ‘They shall look on him whom they have pierced’ – God’s providence decreed that one of the soldiers should open his sacred side with a spear, so that blood with water might flow out to pay the price of our salvation. This blood, which flowed from its source in the secret recesses of his heart, gave the sacraments of the Church power to confer the life of grace, and for those who already live in Christ was a draught of living water welling up to eternal life.
...O soul devoted to God, whoever you may be, run to this source of life and light with eager longing. And with the power of your inmost heart cry out to him: ‘O indescribable beauty of God most high! O pure radiance of everlasting light! O life that gives life to all life! O light that illuminates every light, and preserves in its undying splendor the myriad flames that have shone before the throne of your godhead from the dawn of time!
-- St Bonaventure, The Tree of Life