Couples are asked to contact the parish office soon after their engagement and at least nine months prior to the anticipated wedding date. If there have been previous marriages, dates cannot be set until the annulment process has been completed. Marriage preparation involves four phases: initial interview, marriage inventory and discussion material; sessions with a “lead couple” or Engaged Encounter; and planning the liturgy with the celebrant.
From the United States Conference of Catholic Bishops site
Why does the church teach that marriage is a sacrament?
The sacraments make Christ present in our midst. Like the other sacraments, marriage is not just for the good of individuals, or the couple, but for the community as a whole. The Catholic Church teaches that marriage between two baptized persons is a sacrament.
The Old Testament prophets saw the marriage of a man and woman as a symbol of the covenant relationship between God and his people. The permanent and exclusive union between husband and wife mirrors the mutual commitment between God and his people. The Letter to the Ephesians says that this union is a symbol of the relationship between Christ and the Church.