What the Church actually believes, in the two forms she says it.
The two creeds of the Latin Church: the Nicene, said at Sunday Mass, and the Apostles, said at baptism and on the rosary.
When: Every Sunday, and on any day belief feels thin. Saying it is an act of faith whether or not it feels like one.
Why: A creed is not a feeling. It is what the Church professes, and a person can stand on it when their own conviction is unsteady.