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The gemara in Berachos 8b teaches us: כל האוכל ושותה בתשיעי מעלה עליו הכתוב כאילו מתענה תשיעי ועשירי One who eats and drinks on the ninth day of Tishrei in preparation for the fast the next day, it's as if he fasted on both the ninth and the tenth of Tishrei. Ie that one who eats alot...