Liturgical Note: Why is the Feast of Christ the King Celebrated on Two Different Dates in the Traditional and Novus Ordo Calendars?
The feast was instituted by Pope Pius XI with his encyclical Quas Primas on December 11th, 1925. Dr. Peter Kwasniewski explains in exquisite detail the rationale for the Feast to appear immediately before the Feast of All Saints. See his excellent explanations found here, and here, and here. Needless to say, the "Ultimate" Sunday after Pentecost will be celebrated in the Traditional calendar on November 26th of this year and the following Sunday we will all enter Tempus Adventus.