In addition to the unique prayer services of Rosh Hashana and Yom Kippur, the High Holidays are known for one other service: selichot. A collection of religious poems and verses, selichot are penitential prayers that help one focus on the mood of the season.
An integral part of the selichot service is the repetition of the "Thirteen Attributes of God” (Exodus 34:5-7). After the incident with the Golden Calf, Moses returned to Mount Sinai and assuaged God’s anger at the Israelites. According to the Talmud (Rosh Hashana 17b), God, appearing as a prayer leader wrapped in a prayer shawl, instructed Moses that the Jewish people should recite the following “13 Attributes of God” and they would be granted forgiveness:
Hashem: He is merciful (to one before he/she sins).
Hashem: He is merciful (to the sinner who repents).
Ayl: He is powerful.
Rachum: He is compassionate.
V’chanun: He grants even undeserved favors.
Erech Apayim: He is slow to anger, allowing the sinner time to repent by not exacting immediate punishment.
V’rav Chesed : He abounds in lovingkindness and leniency.
V’emet: He abounds in truth and keeps His promises.
Notzer Chesed La’alafim: He maintains lovingkindness for thousands of generations.
Nosay Avon: He forgives sins that result from temptation.
Va’fesha: He forgives sins of rebellion against Him.
V’chata’ah: He forgives sins committed carelessly or unknowingly.
V’nakay: He completely forgives the sinner who returns to Him in sincere repentance.*
In Sephardi communities, the recitation of selichot begins after Rosh Chodesh Elul and continues through Yom Kippur. In Ashkenzi communities, the recitation of selichot begins on the Saturday night before Rosh Hashana (unless Rosh Hashana begins on a Monday or Tuesday, in which case it begins the previous Saturday night). The first communal recitation of selichot in the Ashkenazi community usually takes place after midnight. On all other days until Yom Kippur, selichot are recited prior to the morning service.
(*Explanations of the 13 Attributes are from The Companion Guide to the Yom Kippur Prayer Service by Moshe Sorscher, printed by Judaica Press.)