Source: How to Help the Holy Souls
To him therefore who knoweth to do good, and doth it not, to him it is sin (James 4:17)
There are a variety of ways to help the Holy Souls. We have listed them in order of effectiveness (i.e. the methods which gain the most grace with the least effort).
Holy Sacrifice of the Mass
The Holy Sacrifice is the greatest help we can offer, because its effect depends on itself, and not on the piety of the priest who offers it...If we cannot have a Mass said, we should at least hear Mass for our dear departed. A Mass has infinite merit...(My Catholic Faith; 1949-1954; Sealed)
In addition to the Requiem Mass following death, a Mass for the departed may be arranged for just about any day of the year. The following are days of traditional significance for Masses for the Departed:
1. All Souls Day
2. Third (Christ's time in the tomb); Seventh (symbolic of eternal repose; St. Ambrose) and 30th (St. Ambrose)from death or burial
3. Yearly anniversary day from death or burial
4. Gregorian Mass: 30 consecutive Masses following the repose (burial) popularized by 6th Century Pope St. Gregory the Great
5. Purgatorian Society Masses: daily masses said for souls those enrolled.
Indulgences are very effective in relieving the Suffering Souls in purgatory.