From a reader:
Can a person in the state of Mortal Sin begin a Novena with the intention of going to Confession as soon as possible? It seems it won’t be efficacious if the soul was in sin when beginning a novena.
No prayer is in vain.
Certainly you can begin a novena of prayers even in the state of sin. That seems like a very good time to start a novena of prayers, as a matter of fact, asking in particular the grace to make a good and thorough confession.
Just do it.