Yizkor (Hebrew, literally "remember") is a traditional mourning service recited by those who have lost a parent or a close loved one. This is based on the Jewish belief in the eternity of the soul and that although a soul can no longer do good deeds after death, it can gain merit through the charity and good deeds of the living. It is recited as part of the prayer service four times during the year.
Yizkor is said following the Torah and Haftarah readings on Yom Kippur, on the last day of Passover, on the second day of Shavout, and on the eighth day of Sukkot (Shemini Atzeret). It is said on Yom Kippur because of the belief that the dead as well of the living need atonement on this day. Yizkor also includes a pledge for charity, which is something that is believed to help avert a harsh decree. The pledge for charity is also fitting for the other three days since the Torah writes (Deuteronomy 16:17) that on the holidays one should come before God with a gift. The holidays are also considered times of charity.
The earliest source for Yizkor is in the Midrash Tanchuma, which cites the custom of remembering the departed and pledging charity on their behalf on Yom Kippur. The Ashkenazi custom of reciting Yizkor on the festivals possibly began during the Crusades when massacres wiped out many Jewish communities.
Yizkor is said by every person who has lost a parent or other loved one. Some authorities hold that one does not say it in the first year following a death. It is the custom in many congregations for those not reciting Yizkor to leave the room during the Yizkor service. In some congregations, Yizkor begins with a series of verses recited responsively. These verses, beginning Adonai, mah adam vatayda'ayhu ("Hashem, what is man that you recognize him") express the mortality of man in comparison to God and relate a trust in God as the owner of man's body and spirit. Some congregations also recite Psalm 91 beginning Yoshev b'seter elyon ("Whoever sits in the refuge of the Most High"), which details God's protection of man.
The central part of Yizkor is a single paragraph beginning Yizkor elohim (may God remember). Prayer books have individualized paragraphs to be recited for a deceased mother, father, male relative (including husband, son, brother, uncle and grandfather), female relative (including wife, daughter, sister, aunt and grandmother), extended family and martyrs. The first four of these paragraphs have a space in which to mention the name of the deceased. They all follow the same pattern: the prayers ask God to remember the soul of the deceased because the one reciting it pledges to give charity on the deceased's behalf. As a reward for this charity, the person asks that the deceased's soul be bound in the "Bond of Life" together with the souls of the forefathers and mothers and the other righteous people in the Garden of Eden. The pledge to charity is included because of the belief that an act of charity will contribute to redeeming a soul, and the prayer essentially asks God to take note of the charity and let it be a merit for the soul of the relative.
After the individuals recite the Yizkor prayer quietly, the Prayer Leader recites another prayer beginning El malei rahamim ("God, full of compassion"), which is similar in content to the Yizkor prayer. This prayer is said on behalf of all the deceased for whom Yizkor was said. This same prayer is recited at funerals and at the synagogue on the anniversary of a family member's death.
The Yizkor service concludes with a prayer called av harachamim, which prays for the souls of all Jewish martyrs. Some congregations specifically mention those who were killed by the Nazis. The regular ayer service continues after Yizkor with another av harachamim, a memorial prayer that is said as part of the weekly Shabbat service.
Sources: Donin, Hayim. To Pray as a Jew: A Guide to the Prayer Book and the Synagogue Service. NY: Basic Books, 1991.
Kolatch, Alfred J. The Jewish Book of Why/the Second Jewish Book of Why. NY: Jonathan David Publishers, 1989.