Translation:On Saturday and Sunday it is the weekend.
As I understand it, the reason it's "er det" instead of "det er" is that a prepositional phrase "på lørdag og søndag " comes first in the sentence. In basic statement-type sentences like this (i.e. not a question or some other kind of sentence) the verb has to be the second major element of the sentence, so the verb "er" comes after the prepositional phrase, and the subject "det" is moved after the verb.
Oh , seriously ? "V2" means "verb 2" and that means the verb is always on the second place in a sentence . Well, I don't know as well as a professional. But as I know , the sentence can be like a question sentence .
Here is the wiki page of it...