The whole arc of the Easter Season leads to Pentecost. And at the center of Pentecost is the Holy Spirit. But with everything that led up to it, it can seem like the Holy Spirit came out of nowhere.
Except he didn't.
The Holy Spirit has always been there, starting with Genesis. And showing us the relationship between the persons of the Trinity, including the Holy Spirit, is one of the reasons for Trinity Sunday.
The other big reason for Trinity Sunday? To show us just how it is that something this old can be a living Faith. How something that is literally universal can at the same moment be entirely personal. How is that even possible?
The Holy Spirit.
This Sunday, get know the person of the Trinity that's closest to your heart.
Readings for Trinity Sunday: http://www.usccb.org/bible/readings/053115.cfm