Who decides what day to celebrate a saint?

The Congregation of the Causes of the Saints is charged with investigating and promulgating the cause of a saint. It is assumed, usually, that a saint is automatically celebrated on the anniversary of their death, as this is seen by Catholics to be that happy occasion when the soul is united with its Creator in heaven. Sometimes, however, there already is a saint, or even several saints on that day, or perhaps it is a major feast day of the Church's liturgical season that would always over-shadow it. In cases such as this, the Church chooses the next most significant day, perhaps the day the saint founded an order, made a great miracle, was baptized, was ordained, etc. Other congregations may be consulted, such as the Congregation of Divine Worship and Discipline of the Sacraments as to the best way the feast may work in the liturgical cycle.