On this Sunday following Pentecost, we celebrate the most Holy Trinity. Actually, every Mass, every prayer, is a celebration of the Trinity: however, it is to our benefit to have a special celebration so that we do not take God for granted.
It is easy to become complacent about God. After all, you are able to contemplate a mystery for only so long! Yet, officially celebrating the Trinity hopefully leads us to the deeper sense of gratitude to our God who has given, and continues to give, us so very much.