Don't no
Regardless of the day of the week, Jews eat only Kosher foods.
Jews eat food -- observant Jews attempt to keep the laws of kashrut by eating kosher food. When? Many Jews eat 3 meals a day with occasional snacks. Some skip breakfast.
no they eat food native to their area
Food, and if they are observant Jews, kosher food. There are two fast days in the Jewish calendar when Jews do not eat anything. Perhaps these are the "non regular days" to which the question refers. These are Yom Kippur and Tisha B'Av.
Jews who eat kosher will not eat snails since snails are not kosher. However, not all Jews eat according to the laws of kashrut, so some Jews do eat snails.
Observant Jews do not eat crayfish.
There is no prohibition against eating kosher meat during Shabbat. The reverse is true; it's expected that meat be eaten during the two main Shabbat meals. On Passover, Jews cannot eat leavened bread (whereas they can at other times of the year). Shabbat does not mirror this. The same kosher rules for the rest of the week apply on Shabbat. Jews cannot eat pork on any day of the week and conversely, Jews can eat tuna or kosher meat on any day of the week.
Muslims and Jews don't eat pork. Muslims eat shellfish, but Jews don't.
Orthodox Jews are only allowed to eat kosher foods.
no, it is because God's Torah forbids Jews to eat pork. Non-Jews are permitted to eat it.
Yes, Jews can eat chicken. Religiously observant Jews would require that the chicken be kosher.
The Torah states that Jews should eat kosher food.