The commandment of Teshuvah—repentance and return to God—is one of the precepts that does not depend on a specific time. The instant a person transgresses, committing a sin, the duty to repent and do Teshuvah for that sin falls upon them immediately. On the other hand, one who has never transgressed or sinned has no obligation to do Teshuvah (in its simple sense), since they have nothing to repent of or amend. In light of the above, it is necessary to understand what Rambam says about the day of Yom Kippur: "Yom Kippur is the time of return—teshuvah—for everyone, for the individual and for the public, and it is the appointed time for forgiveness… therefore, everyone has the obligation to do Teshuvah and confess on the day of Yom Kippur.”.
The question is twofold: those who have sinned must repent immediately, regardless of Yom Kippur; and those who have not sinned—what should they repent about on Yom Kippur? Why does Rambam legislate that "everyone" is obligated to repent on Yom Kippur?
This is where the profound inner essence of Yom Kippur is reflected. The obligation to repent that arises from Yom Kippur differs fundamentally from the obligation to repent as a consequence of sin and transgression. It is a different kind of obligation, and this one applies to everyone.
The duty to repent and return to God throughout the year stems from a person's condition. Those who have sinned have an obligation to return with Teshuvah (repentance), and the degree of Teshuvah is proportional to the sin. Those who have not sinned have no obligation to do Teshuvah. However, on Yom Kippur, the duty to do Teshuvah originates from the very essence of the Holy Day. The essence of Yom Kippur places upon every Jew a general obligation to do Teshuvah—a commandment to do Teshuvah, regardless of whether they have sinned or not.
However, upon the Jew who sinned and has not yet repented and returned to God, Yom Kippur imposes an additional obligation of repentance (Teshuvah)—along with special support from Heaven—derived from the holiness of the day. Meanwhile, upon the one who has already atoned for their transgressions, the Holy Day imposes a new obligation to attain a higher level of repentance.
And this is how Rambam writes: "The sins confessed on this Yom Kippur, one confesses again on the next Yom Kippur, even though one remains steadfast in one's repentance... as it is written: 'My sin is always before me.'" This duty falls upon everyone, even the perfectly righteous (tzaddikim gemurim), since "there is no one on earth who does good and does not sin," and each person must examine their actions and reach a higher level of repentance, a consequence of the very sanctity of Yom Kippur.
The special advantage of Yom Kippur is that it is a time of repentance for all—a special time when the essence of the bond between the Jew and the Most High is revealed, a bond not subject to the limitations and conditions of the world. This bond imposes an unlimited duty to repent—both on the Jew who has not yet atoned for their sins in the literal sense, as well as on the perfectly pious. All can, together, attain on this day a renewed bond with God, a true repentance, and in turn, a Chatima Tova, a final seal for good, for a good and sweet year.
(Likutei Sichot Volume 29, Page 203)
NOTES: 1. As the Rambam writes in the title to the Laws of Teshuvah: "It is a positive mitzvah... that the sinner return from his sin" 2. Hilchot Teshuvah Chapter 2 Halakha 1 3. Ahi, Halakha 8 4. Psalms 51:5 5. Ecclesiastes 7:20