A plenary indulgence, applicable only to the souls in purgatory, is granted each and every day from Nov 1 to Nov 8, who devoutly visit a cemetery and there pray, if only mentally, for the departed.

All Souls’ Day – Plenary Indulgence
A plenary indulgence is granted to the faithful who, on All Souls’ Day, devoutly visit a church or an oratory and recite an Our Father and the Creed

Requirements for obtaining a plenary indulgence:

Do the work while in a state of grace,
Receive Sacramental confession within 20 days of the work (several plenary indulgences may be earned per reception),
Receive Eucharistic communion (one plenary indulgence may be earned per reception),
Pray for the pope’s intentions (Our Father and Hail Mary, or other appropriate prayer, is sufficient),
Have no attachment to sin (even venial) – i.e., it is sufficient that the Christian makes an act of the will to love God and despise sin.