I heard today is the "Feast of the Transfiguration". What is that?
More liturgical Christian traditions than ours put events onto the calendar to remember both heroes of the faith and the major events of Jesus's life. The "Feast of the Transfiguration" on August 6 (for most denominations, not all) is set aside to remember the events of the transfiguration recorded in the synoptic gospels.
Now about eight days after these sayings he took with him Peter and John and James and went up on the mountain to pray. And as he was praying, the appearance of his face was altered, and his clothing became dazzling white. And behold, two men were talking with him, Moses and Elijah, who appeared in glory and spoke of his departure, which he was about to accomplish at Jerusalem. Now Peter and those who were with him were heavy with sleep, but when they became fully awake they saw his glory and the two men who stood with him. And as the men were parting from him, Peter said to Jesus, “Master, it is good that we are here. Let us make three tents, one for you and one for Moses and one for Elijah”—not knowing what he said. As he was saying these things, a cloud came and overshadowed them, and they were afraid as they entered the cloud. And a voice came out of the cloud, saying, “This is my Son, my Chosen One; listen to him!” And when the voice had spoken, Jesus was found alone. And they kept silent and told no one in those days anything of what they had seen.Luke 9:28-36 ESVThis feast is unusual in not focussing on a single character (in this case Jesus), but is seen as a time to reflect on the mystery of the Trinity where God is one being in three persons. God the Father, God the Son, and God the Holy Spirit are all worshipped, celebrated, and remembered in this feast.