From Pieta Prayer Book:
"An indulgence is granted the Christian faithful who devoutly visit a cemetery and pray, if only momentarily, for the dead. This indulgence is applicable only to the souls in purgatory. This indulgence is a plenary one from November 1 through November 8 and can be gained on each one of these days. On the other days of the year this indulgence is a partial one."
"It is, there, a holy and wholesome though to pray for the dead, that they may be loosed from sin." - (2 Mach.12:46).