Catholic's believe that each time Mass is celebrated it is done with the entire Communion of Saints. it would just be ones imagination at work seeing dead relatives at Communion, but in effect I think that is what Catholics believe.
To the offering of Christ are united not only the members still here on earth, but also those already in the glory of heaven. In communion with and commemorating the Blessed Virgin Mary and all the saints, the Church offers the Eucharistic sacrifice. In the Eucharist the Church is as it were at the foot of the cross with Mary, united with the offering and intercession of Christ.
and one of the names for the Eucharist (Communion) comes from the "Communion of Saints"
Holy Communion, because by this sacrament we unite ourselves to Christ, who makes us sharers in his Body and Blood to form a single body. We also call it: the holy things (ta hagia; sancta) - the first meaning of the phrase "communion of saints" in the Apostles' Creed - the bread of angels, bread from heaven, medicine of immortality, viaticum.
So that takes care of the Church Triumphant, but the Church Suffering (the Holy Souls in Purgatory) can also be remembered and had the Sacrifice of Mass offered for their salvation
The Eucharistic sacrifice is also offered for the faithful departed who "have died in Christ but are not yet wholly purified," so that they may be able to enter into the light and peace of Christ:
I believe Orthodox Christians believe the same thing since this is not a point of contention between the churches.