What is meant by the commandment to confess our sins at least once a year? By the commandment to confess our sins at least once a year is meant that we are strictly obliged to make a good confession within the year, if we have a mortal sin to confess.
Confess, therefore, your sins to one another. (James 5:16)
Why should we go to confession frequently? We should go to confession frequently because frequent confession greatly helps us to overcome temptation, to keep in the state of grace, and to grow in virtue.
More on that -> CCC 1440-1449