depends on what kind of fast you are doing, when you are doing a one meal pattern fast you can eat every thing you wish, but make sure you dont overstuff yourself. eat good amount of proteins and carbs. if you are off cereals , than munch oin fruits and make sure your drink plenty of fluids to prevent dehydration, which is common during fasting. you can read this article for further details
http://www.lifemojo.com/lifestyle/fasting-during-navratri-3607943