The Mishnah in Rosh Hashanah 16a says that on the first day of the year (Rosh Hashanah), every human being passes in front of Hashem in judgment. The Gemara there (16b) says in the name of Rabbi Kruspadai that there are three sefarim opened on Rosh Hashanah – one for tzaddikim, one for reshaim, and one for beinonim. The tzaddikim are inscribed for life, the reshaim are written for death, and the beinonim must wait until Yom Kippur to see what their judgment will be. If they are zocheh, they will be inscribed for life; if not, they will be marked for death.
Tosafos there says that the judgments of life and death that the Gemara is discussing refer to life and death in Olam Haba. The Rashba disagrees with this, saying that one is judged only as it pertains to Olam Haba – after one dies, not every year. On Rosh Hashanah we are judged pertaining to the upcoming year.
The Gemara (17b) says that the only thing that an individual can do to be zocheh between Rosh Hashanah and Yom Kippur is teshuvah. The Gemara says that even if he brings all of the world’s korbanos, he will not be zocheh unless he performs teshuvah.
The Rishonim were bothered by this. They asked why it should not suffice for one to perform any mitzvah in order to tip the scales in his favor. Why is it necessary for one to perform teshuvah over the performance of any other mitzvah? The Ritvah quotes from Tosafos that in fact one can do any mitzvah, and if the amount of mitzvos outnumbers his aveiros he will be judged favorably.
The Ritvah quotes Rabbeinu Yona’s explanation that the reason why one must do teshuvah is that Hashem commanded us to do teshuvah at this time, for as the pasuk says: “lifnei Hashem titharu.” Therefore, Hashem gave us one opportunity to help our din: do teshuvah. Rav Itzela explained that not doing teshuvah during this time, when it is so accessible, is a greater sin that would outweigh any mitzvah.
The Emek Berachah explains that indeed after Rosh Hashanah the din is sealed and one is only judged on his actions of the previous year. Any actions performed after Rosh Hashanah will be on the following year’s cheshbon. The reason why one can still perform teshuvah after Rosh Hashanah is because teshuvah erases aveiros retroactively. When one does teshuvah on the aveiros performed during the previous year he can erase aveiros from the scale and thus receive a favorable judgment.
The Rambam (Hilchos Teshuvah 3:4) writes that the message of the shofar is to wake up those who are sleeping and get them on the right path. Each person should view himself as if he has half mitzvos and half aveiros – and therefore try to perform many more mitzvos. The Rambam concludes by saying that it is for this reason that klal Yisrael performs many ma’asim tovim and gives tzedakah during the Asseres Yemei Teshuvah. It seems from the Rambam that one can tip the scales in his favor by performing any mitzvah.
However, the sefer, Harirei Kedem, explains that the reason why the Rambam said that we should do many mitzvos is not because those new mitzvos will tip the scale in our favor; rather, it is part of the teshuvah of Rosh Hashanah. We find that the teshuvah of Rosh Hashanah differs from that of the rest of the year. Generally, when one wants to do teshuvah on an aveirah there is a four-step process that is performed: vidui (confession), charatah (regret), azivah (stopping oneself from sinning again), and kabbalah (resolution).
But we do not find that any of these processes are performed on Rosh Hashanah, nor are they part of the davening. This is because the teshuvah of Rosh Hashanah is the first step in the process; it is the process of waking up and changing direction. Thus, the Rambam writes that one should perform mitzvos between Rosh Hashanah and Yom Kippur to show that we have turned our lives around. We want to show Hashem that we have changed our direction and will begin performing more mitzvos. While these new mitzvos do not add to the side of the mitzvos on the scale for the previous year, they show Hashem that we have begun to change.