חסידות אויף זכריה 12:3

This is the aspect of the yahrtzeit, which is the day that one's mother or father died and on which one says Kaddish (Shulchan Aruch Yoreh De'ah Siman 376, Seif 5 in the Rama) and fasts (Shulchan Aruch Orach Chaim Siman 568, Siman 8). For then, the children engage in rectifying and elevating the souls of their mothers and fathers who have died. Death is the aspect of sleep. And when one rectifies and elevates them this is the aspect of resurrection of the dead, which is the aspect of awakening from sleep, the aspect of (Daniel 12:2) "Many of those that sleep in the dust of the earth will awake", the aspect of "and keeps Your faith to those that lie in dust" (Liturgy, the Amidah). Therefore the blessing of "resurrecting the dead", מְחַיֵּה הַמֵּתִים, begins "אַתָּה גִבּוֹר לְעוֹלָם אֲדֹנָי", "You are mighty forever Hashem". Because the essence of the resurrection of the dead is the aspect of renewal of the world through prayer in the aspect of justice, as seen above. This is the aspect of 'You are mighty', etc. (And as explained in another place, that all prayers require evoking the strength of the prayer, in the aspect of judgments, as seen above, which is the blessing 'You are might', etc.). Therefore one says Kaddish at that time, because the Kaddish is a very awesome and wondrous praise, as explained, 'for it breaks all bolts of iron', etc., as it written in the Holy Zohar (Terumah 129) because Kaddish evokes the aspect of song of the future. Therefore the Kaddish begins 'in the world which He created according to His will'. In other words, the aspect of faith in the renewal of the world, which we believe that He created the world according to His will. 'May He establish His kingdom'; which is the aspect of the renewal of the future world. For then His Kingship will be revealed for all to see, as it is written (Zechariah 14:9) "And Hashem shall be King over all the earth; in that day...", etc. And this is 'Amen, May His great name be blessed for ever, and to all eternity, Blessed and praised', etc. 'above and beyond all the blessings, hymns, praises and consolations'. All of this is the aspect of the song of the future, as seen above, which is 'above and beyond all the blessings', etc. Therefore the children say this awesome prayer for the souls of their mother and father, to elevate them and rectify them. For the essence of their rectification is in the aspect of resurrection of the dead, the renewal of the world. This is awakened by the awesome prayer of the Kaddish, which is a song of the future, as seen above. This is the happiness that is hoped for. For then one recognizes Hashem Yisbarach and knows what the kosher tzaddikim achieved through their service. Happy is he who waits for this!