see this post.
There is a diyuk in Rashi near the beginning of Kesuvos if the night of the 9th of Tishrei is also a mitzvah to eat, or is it only a mitzvah on the day of the 9th (i.e., from the time you wake up on erev yom kippur, but not the night before).
I saw a nice thought in the name of Rav Kook about this - he said the fasting on Yom Kippur represents teshuva in an ascetic state - through fasting and prayer, and the mitzvah to eat on Erev Yom Kippur represents Teshuva in a non-ascetic state.
The need for both of these are true, and his point is correct. I just think that eating on Erev Yom Kippur is a pre-ascetic state. I think if we break our fasts in a refined manner, that would be teashuva while in a non-ascetic state.
I've said many times, Halevai people should eat the matza on the first seder with the same enthusiasm as they eat their first piece of chametz on motza'i Pesach.
Gmar Chasima Tova.