Mass can be celebrated for the living members or those of who have passed away.
Why? Because it is the eternal merits won for us by Christ on the cross that are applied to the soul who is either alive or dead. We are one big church with three divisions.
The first is Church Triumphant. This church consists of all the angels and saints who sit at the eternal banquet which Christ prepared for them in paradise. It is very much an interactive community of joy.
The second is Church Suffering. This church consists of the Holy Souls of purgatory. These souls died as friends of God, but needed further purgation or cleansing because they either didn’t finish their penance from their forgiven sins or still had earthly attachments that they needed to work out. Last is Church Militant, the baptized faithful who keep up the fight of good against evil.
All three churches are united in praise around the altar at Mass. Since there is a suspension of time and space at Mass, Calvary is brought through the ages and every soul, either in heaven, in purgatory, or on earth at Mass, offers praise and worship of the Triune God. Mass is the most perfect prayer. It is the offering of the eternal Son to the Father in union with the Holy Spirit on behalf of the faithful. The fruits of the Sacrifice are the grace of the sacrament.
The saints in heaven and the faithful on earth can help the holy souls of purgatory by their prayers. The saints can intercede to God on their behalf, and the faithful, through the mediation of the priest, can offer the Mass on their behalf. The infinite merits that Christ won on the cross are applied to their souls during the Mass. The spiritual graces then aid the holy souls in purgatory and reduce their time there.
This is why Catholics will seek an intention for the Mass for the faithful departed. Usually this is done on their birthdays and the anniversaries of their passing. If a certain Mass is being celebrated for a deceased person and this person is indeed in heaven, the fruits are not wasted. Rather, the next soul in purgatory will then benefit from this Mass.
Masses can also be said for the living. One Mass celebrated in someone’s honor is worth many after they die. It is customary to have a Mass celebrated in honor of a birthday, wedding anniversary, or some other joyous occasion.
What a beautiful gift to give someone the Holy Sacrifice of Jesus for their intentions.