In the Roman Catholic Church, priests wear the color green on every Sunday during Ordinary Time.
Nicholas Gihr in his book The Holy Sacrifice of the Mass brings to mind the images of a tree and garden to explain the symbolism of the color green:
Green is in harmony with the very nature of the Church—she is a mighty tree, which lifts its top majestically toward heaven, spreads its shady branches and leaves in benediction over the earth, resplendent with the richest blossoms, bringing forth choice fruits of grace and virtue u in abundance. She is the watered garden of the Lord; Christ, the good shepherd, leads his sheep to ever green pastures. The Church clothes herself in green vestments to express her joyous, lively hope of the ever lovely and eternally verdant meadows of the heavenly paradise, of the incorruptible inheritance and the unfading crown of glory in Heaven.
In this way the color green urges us to grow in our spiritual life that we may be led into the Promised Land of Heaven.
This color is used until the First Sunday of Advent, though the last Sunday to use that color is the 33rd Sunday of Ordinary Time.
Solemnity of Christ the King
When the world was in turmoil and the increasing growth of secularism was spreading throughout the world, Pope Pius XI sought to combat it with an affirmation that Jesus Christ is the true king to whom we owe allegiance.
He did this by establishing a feast in honor of “Our Lord Jesus Christ the King” in 1925, through his encyclical letter Quas Primas. Pius XI explained, “We firmly hope … that the feast of the Kingship of Christ, which in future will be yearly observed, may hasten the return of society to our loving Savior.“
That feast eventually replaced the 34th Sunday of Ordinary Time and the priest always wears white or gold on that Sunday, as it is a special feast dedicated to Jesus Christ.
Priests continue to use green during daily Mass, up until the start of Advent, but the last Sunday to feature that color is the 33rd Sunday.
And today we celebrate...