Erev Yom Kippur

"Anybody that eats on the ninth shall be considered as one who has fasted on the ninth and tenth" (paraphrase) This simply means that if you prepare yourself before Yom Kippur, with the ninth referring metaphorically to the days leading up to Yom Kippur and eating referring metaphorically to the preparation, it will be considered as if you had an extended Yom Kippur consisting of the preparation time too. "Me shetorach be'erev Shabbos Yochal bi'shabos". As if you fasted, referring metaphorically to the goal that you trying to achieve and just like Yom Kippur you are achieving that goal, so to, on the days before you can achieve that goal by preparing. We eat a large feast on Erev Yom Kippur to drive that point home.