Is this correct?
Ich freue mich, dass es Wochenende ist.
Or should I say:
Ich freue mich, dass es das Wochenende ist.
With or without the article? I have definitely heard the first, but I am not sure if it is informal/slang.
In your example it is absolutely correct to leave out the article. The pronoun "es" is used here as a placeholder to further specify a following condition. We can better see the grammatical construction if we split the subordinate clause:
In this case "Wochenende" is the condition for "es" as it would also be the case with other conditions like e.g.
Note that in case we further elaborate the condition (e.g. by using an adjective) we then need an article:
"Ich freue mich, dass es Wochenende ist" is correct, but the most idiomatic form is without "es": "Ich freue mich, dass Wochenende ist".
"Das Wochenende" in this context would always refer to a specific weekend (e.g., the first weekend in April), never to weekends (or whatever happens to be the current weekend) in general.
I'm not sure, but I have the impression