It is not the definite form of helg in Swedish. Det is just a placeholder pronoun here like it in it is raining. We would say Det är helg på lördag, not helgen.
The V2 rule – the verb needs to be in second place (except in questions and subclauses). 'På lördag' is considered one unit, so the verb must go right after that.
Why wouldn't helgen be used? Wouldn't we need to say that on Saturday it's the weekend, rather than just it is weekend?
No, we think of this as a general thing – we don't mean that it is one specific known weekend, we mean that 'weekend' is a characteristic of the day, or something like that. Just like we say that tomorrow is onsdag, not onsdagen (it's Wednesday), that det är vinter not det är vintern (it's winter), and so on.
I'm far from the expert in Swedish, but we do similar things in English. We usually say "it is summer", not " it is the summer"
I wrote "On Saturday it is a holiday". Marked wrong. Suggested answer: "On Saturday it is holiday". This is incorrect English. It needs an article.
Why is "det" used here ? Is it because "är" is the verb and "helg" is the direct complement and it needs a subject?
Where is this "verb 2" rule I keep hearing about? What is this word order? Where is the explanation??