According to canon 527 in the Code of Canon Law, there are three parts to a priest becoming pastor of a parish. First, the diocesan bishop must appoint the priest. This occurred back in the spring when I received my letter of appointment from Archbishop Cupich on April 22. He wrote, "Father Wallace: With this letter, I am pleased to appoint you Pastor of Saint Juliana Parish in Chicago. This appointment, effective July 1, 2016, is made in consultation with the Diocesan Priests' Placement Board, as well as with your Vicar and Dean. Your term of office will continue until you are reappointed, transferred, or your successor is named. Bishop Francis J. Kane will act as my delegate in receiving your Profession of Faith."
The second part of the process of becoming a pastor is the aforementioned Profession of Faith, which I will make before you all this Saturday evening. The third is the priest ‘taking possession’ of the office. I took possession on July 1st when I moved to the parish. The Mass of Installation is a more formal and solemnized expression of the parish being entrusted to me.