We celebrate John’s birth because he is such an important person in the NT. Only the birthday of Jesus and Mary are also celebrated by the church; this fact alone shows the importance of John.
John is the precursor of Christ, the one who comes to prepare the way for the Lord. He takes his mission very seriously. There is something attractive about the desert for people who want to follow a strict and harsh spiritual life, and it is to here that John retires when he wants to prepare for his ministry.
He does his preparation well and soon he begins to preach and draw large crowds. The Jews have waited hundreds of years for their Messiah and John knows that he is not the Messiah but the one who prepares the way. In all things he points to Jesus.
But he also tells us to prepare our souls to receive Jesus: he calls us to repent so that we can have the open heart that is necessary to hear the word of God in the midst of all the noise of the world. This is important for often we are caught up so much in being a Catholic that we miss what it is to know Jesus: we do the rituals and obey the laws in the same way as the Pharisees, and miss the spirit of the teaching of the Lord.
Thankfully, every now and then in life the desert catches up with us and we are forced to look at our real relationship with Jesus; such moments are moments of blessing.