During the ten days between Rosh Ha-shana and Yom Kippur, God creates life for the upcoming year. So that His kindness does not reach the wicked, He judges all His creations during these days. Since He wishes to benefit all His creatures, and especially His people, through whom He channels shefa and blessing to the world, He is close, during this period, to all who sincerely call out to Him, and He extends His hand to penitents. Therefore, though repentance is always welcome, during these ten days it is more pleasing and is immediately accepted, as we read (Yeshayahu 55:6), “Seek the Lord while He can be found; call to Him while He is near” (Rosh Ha-shana 18a; MT, Laws of Repentance 2:6). During this period, individual repentance is accepted as readily as communal repentance (Pesikta Rabbati).
Therefore, it is proper during this period for a person to examine his actions in order to repent and correct his misdeeds from the past year (Rema OḤ 603:1). One should especially make an accounting of interpersonal matters, as Yom Kippur does not atone for interpersonal sins unless one first placates the person he offended (ibid. 603:4). If one has a disagreement with another about money, he should not decide for himself that he is in the right, as self-interest may blind him. Rather, he should approach a rabbi and ask for guidance. Before going to sleep each night, the pious engage in soul-searching, confess their sins, and return to God (Zohar III, 178a). During the Ten Days of Repentance, everyone should do so (MB 603:2).
Since it is primarily at this time that the world and humanity are judged, Jews customarily take special care to avoid anything prohibited and to increase Torah study, prayer, charity, and good deeds. Additionally, it is customary to wake up early to recite Seliḥot and supplications in the synagogue (MT, Laws of Repentance 3:4).
It is said in the name of Arizal that the seven days between Rosh Ha-shana and Yom Kippur contain one of each day of the week, so on each day, one should try to improve himself through repentance, Torah study, and good deeds, to rectify whatever spiritual damage he did on that day throughout the year. On Sunday he should correct his misdeeds of Sundays, on Monday of Mondays, and so on (Sha’ar Ha-kavanot 90:3; MB 603:2).
Some have a custom to be more meticulous about various areas of halakha, like kashrut, during this time period (SA 603:1 based on y. Shabbat 1:3). People who rely on a lenient minority opinion during the year, due to various pressures, observe the proper, le-khatḥila practice according to most poskim during this period. For example, during this period it is proper to eat only glatt meat and wash mayim aḥaronim before reciting Birkat Ha-mazon.
The Sages said that one should always view himself – and the world as a whole – as being halfway between guilt and innocence. Since both the world and the individual are judged in accordance with the majority of deeds, a person doing one mitzva can tip the scales toward the side of merit for himself and for the whole world. And woe unto him who does one sin; he may have tipped the scales toward the side of guilt for himself and for the whole world. We read (Kohelet 9:18), “A single sin destroys much good.” One sin can withhold so much good from the perpetrator and from the whole world (Kiddushin 40b).