We celebrate Cristo Rey or the Feast of Christ the King because we as firstborn Christians(Yes, because Catholics are firstborn Christians ) know that Jesus Christ reigns the world and this universe. We also believe that He will come again to judge us according to what we have done. 

Protestants like Lutheran, Anglicans, and even the neo-Protestants also celebrates this but unlike Catholics that we are worshipping a Living Christ not the “imagenary” one. 

It is also called as The New Year of the  Church because it is the first day of another liturgical year and the first day of the season of Advent or the coming of the Word became Flesh and Dwell among us, the Emmanuel, Jesus Christ.
Photo: Catholic Fortress