YOM KIPPUR

We have three days to completely purify ourselves, "Before Hashem you shall be purified" (Leviticus 16:30), 'you shall be purified' has the numerical value of 'kether'.

Rabbi Yehuda HaNasi says that the essence of the day atones, but only on the condition that one accepts not to commit more transgressions.

The essence of the day atones, and any transgression, even if a person stumbles on Yom Kippur, if he intends to return to the transgression, it is immediately atoned for him.

So according to this, when is one liable for karet?

Even from karet he is exempt, so only for the intentional sins, only if he does it at the last moment when he can no longer do teshuvah on Yom Kippur.

Only if he sinned, God forbid, at the very last moment before the stars appear, the day does not atone, and he needs to do teshuvah, and then everything is atoned.

Edited from 'Shivivei Or'