I translated "On Saturday we eat chicken" as "Sabato mangiamo pollo", but it was marked wrong because I didn't use the article - il sabato. I haven't seen articles used with days of the week anywhere else; could someone explain why the article is necessary here?
with days of the week it is used to indicate something that recurs on the same day--a habitual action. The meaning is that on each Saturday we eat chicken, as opposed to something that just happens on one Saturday.
When exactly do you use "lo" instead of "il/la"? I thought you replaced "il/la" with "lo" when the noun being described begins with "z" or "s". "Sabato" begins with an "s" but its definite article is "il", not "lo"...?