i don't know if you still need this answer, but you can use the pot whenever (once food is introduced)! i just recently figured out that using the pot for an egg and then the pan for bread is how you can make an egg toast order since they'll cook at the same time.
while i was playing, i saw that food was introduced by day 3! if it hasn't been introduced by then, it may be based on your performance (how much money you make and/or your grand total)?? but i can't say that for sure--just a guess!
