In the generation of the Exodus from Egypt, Moses ascended Mount
Sinai three times. The first was to receive the Torah.
The second was to plead with G-d for his forgiveness, after the
Jewish people sinned in worshipping the golden calf.
Then, on the first day of Elul -- the month immediately preceding
Tishrei -- Moses ascended the mountain a third time, to invoke G-d's
abundant mercy for our complete atonement. He remained there for
forty days, until Yom Kippur, when G-d cleansed us completely, as
though we had never sinned.
Since then, these days are marked as a special period of Divine
Grace, during which our sincere prayers are sure to find favor
in the eyes of G-d.