Few teachings of the Church have been more misunderstood. The mystery of the Holy Trinity, Father, Son, and Holy Spirit, is the central mystery of our Catholic faith. Its really good to have a special day set aside to realize the importance of this critical aspect of faith.
With all the philosophical knots the idea of the Trinity can tie your brain in, it can be easy to forget how simply it can be demonstrated. One of my favorite saints, as well as one of my patrons, St. Patrick used a humble shamrock to explain the Holy Trinity to the people of Ireland. Sometimes showing is more useful than trying to intellectualize. God made the Trinity hard to explain, but easy to see. Just look at a patch of clover and you'll see it.