What?
Selichot are special prayers said by Jewish people requesting forgiveness from G-d.
When?
Selichot are said are on fast days as they are days on which Jewish people request forgiveness. They are also said in the period leading up to Yom Kippur which is the holiest day in the Jewish calendar; the day on which the Jewish people pray all day before G-d and show desire to draw close to Him.
According to Sephardic tradition, Selichot are said from the beginning of Elul, the month before the Days of Awe, up until Yom Kippur. Ashkenazic tradition is such that Selichot are recited ten times up until and including Yom Kippur. This custom is such due to a once wide-spread custom whereby Jews would fast for ten consecutive days (they would eat at night) up to and including Yom Kippur.
There are ten days between and including Rosh Hashanah and Yom Kippur and on four of those days it is inappropriate to fast; the two days of Rosh Hashanah, the Shabbat between Rosh Hashanah and Yom Kippur and the eve of Yom Kippur. It was agreed that a particular day of the week should be identified as the one on which the recital of Selichot start which leaves two possibilities; the Saturday night immediately before Rosh Hashanah and the Saturday night of the preceding week.
The proper time for saying Selichot is at the end of the night just before morning. The reason for this is that Jewish mysticism views this time as being an especially favorable time with regard to the presence and closeness of G-d. Typically, Selichot are recited early in the morning before the daily morning prayers.
On the first night of Selichot the custom is to say them before going to sleep and since the first part of the night is considered a time of judgment (in Hebrew “din”), Selichot aren’t said until after what is known in Hebrew as “chatzot” which is midnight according to the Jewish division of hours. The time of “chatzot” varies from location to location around the world, so it is advised to consult a Jewish calendar or a local Rabbi to determine the time of “chatzot” in one’s specific area.
The Thirteen Attributes
The thirteen attributes are a fundamental part of the Selichot service. The Thirteen Attributes are a list of G-d’s thirteen attributes of mercy that were revealed to Moses by G-d after the Jewish people sinned with the Golden Calf. The Thirteen Attributes are G-d’s reassurance to the Jewish people that repentance is always possible and that G-d awaits the repentance of each and every Jew, no matter how far they are from Him. If we try to emulate G-d’s merciful ways, He too will be merciful in return.
Source by Uriel Sela