Divine Mercy Sunday is celebrated annually on the first Sunday after Easter. On this day, you can receive a Plenary Indulgence! A Plenary Indulgence removes all temporal punishment due to forgiven sin ("Temporal punishment" is the "making up" for sin to which we are obliged after we are forgiven). A Plenary Indulgence can be applied to one's self or to anyone who is deceased. A Plenary Indulgence can be gained once per day.
To learn what a Plenary Indulgence is, as well as "temporal punishment", click here.