This Motzai Shabbos (early Sunday morning) we begin saying Selichos. These special prayers are intended to guide us to thoughts of repentance as we approach Rosh HaShana.
Selichos are said for a minimum of four days, but if Rosh HaShana falls on a Monday or Tuesday (like this year) we begin the Sunday before.
Why four days and why Sunday?
Traditionally, pious people would fast ten days for the Ten Days of Repentance but since one may not fast on the two days of Rosh HaShana, Shabbos, and Erev Yom Kippur, they would begin Selichos (and fast) four days earlier.
Additionally, we are to look at ourselves as korbanos, sacrifices, (literally things which are brought close) and an animal brought for sacrifice was checked for blemishes four days prior to its offering. We, too, are to use these days to introspect.
We begin on Saturday night/Sunday because the Creation of the world began on Yom Rishon, and culminated with the creation of Man, which is what we celebrate on Rosh HaShana as we try to recreate ourselves in Hashem’s image once again. Starting Motzai Shabbos also enables us to use the joy of Shabbos as a launching pad for our return to Hashem, providing a powerful boost.
Teshuva isn’t scary, it’s a gift; a chance to repair our relationship, and that takes time.