While the Great Jubilee celebrates the new Millennium in the year 2000, the reasons are not secular. The Church is not concerned with the change of the calendar, per se. The Great Jubilee is for the Church a celebration of the Incarnation of Jesus Christ, and ultimately of the Redemption, both of which occurred in "the fullness of times."

Gal 4:4-6 But when the time had fully come, God sent forth his Son, born of woman, born under the law, ⁵ to redeem those who were under the law, so that we might receive adoption as sons. ⁶ And because you are sons, God has sent the Spirit of his Son into our hearts, crying, Abba! Father!

John 1:1-5, 10-14 ¹ In the beginning was the Word, and the Word was with God, and the Word was God. ² He was in the beginning with God; ³ all things were made through Him, and without Him was not anything made that was made. ⁴ In Him was life, and the life was the light of men. ⁵ The light shines in the darkness, and the darkness has not overcome it.

¹⁰He was in the world, and the world was made through Him, yet the world knew Him not. ¹¹ He came to His own home, and His own people received Him not. ¹² But to all who received Him, who believed in His name, He gave power to become children of God; ¹³ who were born, not of blood nor of the will of the flesh nor of the will of man, but of God. ¹⁴ And the Word became flesh and dwelt among us, full of grace and truth; we have beheld His glory, glory as of the only Son from the Father.

All of this occurred in keeping with an Eternal Plan of the Heavenly Father:

Eph. 1:3-10 ³ Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, ⁴ even as He chose us in Him before the foundation of the world, that we should be holy and blameless before Him. ⁵ He destined us in love to be His sons through Jesus Christ, according to the purpose of His will, ⁶ to the praise of his glorious grace which He freely bestowed on us in the Beloved. ⁷ In Him we have redemption through his blood, the forgiveness of our trespasses, according to the riches of His grace ⁸ which He lavished upon us. ⁹ For He has made known to us in all wisdom and insight the mystery of His will, according to His purpose which He set forth in Christ ¹⁰ as a plan for the fullness of time, to unite all things in Him, things in heaven and things on earth.

Furthermore, it is the "working out" of the Father's Plan in we who believe:

¹¹ In Him, according to the purpose of Him who accomplishes all things according to the counsel of His will ¹² we who first hoped in Christ have been destined and appointed to live for the praise of his glory. ¹³ In Him you also, who have heard the word of truth, the gospel of your salvation, and have believed in Him, were sealed with the promised Holy Spirit, ¹⁴ which is the guarantee of our inheritance until we acquire possession of it, to the praise of His glory.