There is no other Sunday like Trinity Sunday. A festival, a celebration not of some past event or important day in God's plan of salvation. A day dedicated simply to the wonder and greatness of God. Our God who is described in complete unity throughout the Bible as three-in-one, Father, Son & Holy Spirit.
Rather than try to comprehend the incomprehensible of how this can be, let's marvel at the wonderful love, forgiveness and blessings of God.