It is written that the first day of Succot (Feast of Tabernacles) is sometimes called the first day of forgiveness.
Why is this so?
Rabbi Yitzhak of Wurka, a Chasidic rabbi of some standing, is said to have given the following answer from his own experience:
"I used to be a treasurer for a large estate and once, as I was closing the books for the year, it appeared that it had been a relatively bad year.
I simply delayed the final statement for a couple of weeks, knowing that a particularly profitable deal was going to change the final reckoning."
So, too, if we were to make our accounts with the Lord in the month of Elul, we would most likely be at a disadvantage.
If, however, we postpone closing the ledger for a few weeks until after Rosh Hashanah and Yom Kippur, when we repent and atone for our sins, and then wait a couple of days for the first day of Succot, during which time we have little occasion for anything but the performance of mitzvot, then we have that much more chance of showing a profitable year.
We can then begin a new account for the next year."
–Rabbi Adin Steinsaltz
From "The Protective Power of Prayer," In the Beginning by Rabbi Adin Steinsaltz