Could "helget" be used instead of "det helg"?
No, as helg is masculine, the definite form is helgen. Here, det means it and is not the article.
Because the verb must be in second position, 'på lørdag er det helg' ('på lørdag' taken as one because of it being a preposition phrase) is literally 'on Saturday is it weekend' but the definite article is not needed here.
Why not det er helg?
Because the verb goes in the second position in Norwegian.