Canterbury Tales by Dr. Taylor Marshall: Why Divine Mercy Sunday is Better Than a Plenary Indulgence: With a plenary indulgence the usual conditions are:
1) Receive Holy Communion
2) Make a good Confession within 20 or so days
3) Pray for the Pope
4) Be fully detached from all sins
The last one (4) is the most difficult and most subjective. No one knows if they have attained full detachment from all sins. Hence, plenary indulgences are difficult and uncertain.
The amazing thing about Christ's promise to us on Divine Mercy Sunday is that the condition for detachment of sin is absent. This means that if you make a good confession and receive Holy Communion devoutly, you will receive full remission of all temporal punishment. If then you went to Confession and died after receiving Communion on Divine Mercy Sunday you would not spend one single moment in purgatory!!!