Originally, the Selichot service consisted of several groups of biblical verses, each culminating in the recitation of the Lord’s 13 attributes enumerated in Exodus 34:6. In Geonic times (the sixth to the 12th century), the service was expanded and enriched with the inclusion of the penitential prayers written by liturgical poets.
Selichot: Prayers of Repentance
Selichot, prayers for forgiveness, are ancient prayers already mentioned in the Mishnah. They originated as prayers for fast days. The Mishnah describes public fast days and the order of prayer for such occasions as featuring a series of exhortations that end with the words "He will answer us," recalling the times in Jewish history when God answered those who called upon Him. The Tanna deve Eliyahu Zuta, a midrashic work that dates at the latest to the ninth century, mentions a special service for forgiveness instituted by King David when he realized that the Temple would be destroyed. "How will they attain atonement?" he asked the Lord and was told that the people would recite the order of Selichot and would then be forgiven. God even showed David that this act of contrition would include a recitation of the "Thirteen Attributes of God," a descriptive passage from Exodus that expresses God's merciful nature:
"The Lord! The Lord! A God compassionate and gracious, slowto anger, rich in steadfast kindness, extending kindness to the thousandth generation, forgiving iniquity, transgression, and sin; yet He does not remit all punishment..." (Exodus 34:6‑7).
The name "Lord" [the Hebrew letters YHWH which constitute God’s name] was consistently understood by the Rabbis as referring to the appearance of God in His attribute of mercy, Therefore, its repetition in this passage indicated that God was merciful at all times. As the Talmud put it:
"The Lord! the Lord!"‑-I am the same before one sins and after one sins and repents. "A God compassionate and gracious..." Says Rabbi Judah, "A covenant has been made concerning these Thirteen Attributes. They will never be turned away empty handed..."
The Selichot service also emphasizes the recitation of "The Thirteen Attributes." Over the centuries, special poems embellishing this passage were added to the Selichot. The exact poems to be recited may differ from place to place, but the basic elements of the service have remained the same throughout the Jewish world. Because of its emphasis on God's forgiving nature, this text describing "The Thirteen Attributes" plays an important role in the Yom Kippur liturgy as well.
The tradition of reciting Selichot throughout the month of Elul may stem from the fact that it was customary to fast six days before Rosh Hashanah. Since the Selichot originated as prayers for fast days, it followed naturally that they would be recited at this time.
Tomado de My Jewish Learning
El movimiento reformista ha editado hace años un servicio especial de Selichot. En Google Books se puede acceder a parte de la obra: