The Solemnity of Our Lord Jesus Christ, King of the Universe is Sunday, November 21st, 2021.  This is celebrated on the last Sunday in Ordinary Time.
 

What is the Solemnity of Our Lord Jesus Christ, King of the Universe?

Pope Pius XI instituted this feast in 1925 with his encyclical Quas Primas (“In the first”) to respond to growing nationalism and secularism. He recognized that these related societal ills would breed increasing hostility against the Church.

Today reminds us that while governments and philosophies come and go, Christ reigns as King forever.

During the early twentieth century, in Mexico, Russia, and in many parts of Europe, atheistic regimes threatened not just the Catholic Church and its faithful but civilization itself. Pope Pius XI’s encyclical gave Catholics hope and—while governments around them crumbled—the assurance that Christ the King shall reign forever. Pope Pius XI said that Christ “reign[s] ‘in the hearts of men,’ both by reason of the keenness of his intellect and the extent of his knowledge, and also because he is very truth, and it is from him that truth must be obediently received by all mankind.”

Quas Primas continues to ring true. In recent years, aggressive secularist campaigns have sought to marginalize the Church and other religious institutions. In response to alienation and loss of solidarity – which tend to accompany these secularist assaults – nationalistic movements have sprung up across the western world. Now, as always, we must turn and gaze on the face of Christ, our merciful Lord.

-United States Conference of Catholic Bishops

What is the encyclincal (Papal Letter) on the Solemnity of Our Lord Jesus Christ, King of the Universe?

It has long been a common custom to give to Christ the metaphorical title of "King," because of the high degree of perfection whereby he excels all creatures. So he is said to reign "in the hearts of men," both by reason of the keenness of his intellect and the extent of his knowledge, and also because he is very truth, and it is from him that truth must be obediently received by all mankind.

He reigns, too,  in the wills of men, for in him the human will was perfectly and entirely obedient to the Holy Will of God, and further by his grace and inspiration he so subjects our free-will as to incite us to the most noble endeavors.

He is  King of hearts, too, by reason of his "charity which exceedeth all knowledge." And his mercy and kindness which draw all men to him, for never has it been known, nor will it ever be, that man be loved so much and so universally as Jesus Christ. But if we ponder this matter more deeply, we cannot but see that the title and the power of King belongs to Christ as man in the strict and proper sense too. For it is only as man that he may be said to have received from the Father "power and glory and a kingdom," since the Word of God, as consubstantial with the Father, has all things in common with him, and therefore has necessarily supreme and absolute dominion over all things created.

-Pope Pius XI, Quas Primas ("In the First")