The Transfiguration of Christ is the culminating point of His public life, as His Baptism is its starting point, and His Ascension its end.
About a week after His sojourn in Cæsarea Philippi, Jesus took with him Peter and James and John and led them to a high mountain apart, where He was transfigured before their ravished eyes. St. Matthew and St. Mark express this phenomenon by the word metemorphothe, which the Vulgate renders transfiguratus est. The Synoptics explain the true meaning of the word by adding “his face did shine as the sun: and his garments became white as snow,” according to the Vulgate, or “as light,” according to the Greek text.
This dazzling brightness which emanated from His whole Body was produced by an interior shining of His Divinity.
Catholic encyclopedia on “Transfiguration”: https://www.newadvent.org/cathen/15019a.htm
The feast was present in various forms by the 9th century, and in the Western Church was made a universal feast on August 6 by Pope Callixtus III to commemorate the lifting of the Siege of Belgrade (1456).