The day before Rosh Hashanah is the last day of the year, and it is the last opportunity to repent before the Day of Judgment. It is a day of reflection on the year gone by, and a day of preparation for the coming year.
The first two forty-day periods of prayer were of no avail, and it was only after an additional forty days that God said to Moses "I have forgiven according to your request." Therefore, we recite selichot during this same last period of forty days.
Though God accepts the repentance of his children all year long, the forty days between Rosh Chodesh (New Month) Elul and Yom Kippur are more suited for repentance than any other period of the year, for they are days of Divine mercy and favor.
Some people have certain prayer customs on Rosh Hashanah and Yom Kippur which are not accurate according to Jewish law. All the same, a person should not hastily renounce a custom which has be handed down from generation to generation.
Blowing of the Shofar, Lulav, and Megillah on Shabbat.
Halachic Reasoning.
Conceptual Reasoning.
The Sacrifice: The Difficult Test.
Why spread out the many reasons.
The force of repentance with the Torah.