i still don't have the point when do we need the article before the days (lunedi, martedi etc.)
My understanding of this is that you need the article when you want to say that something happens on a certain day. eg "succede il lunedì" = "it happens on Monday" or "it happens Mondays" Hope this helps.
Why do we need the è?
If you mean the grave accent, it differentiates "è" (is) from "e" (and). Otherwise we need a verb to form a sentence and the verb here is essere (to be) in the 3rd person singular form "è" (is).
Date: Sunday March 20 2016
Does this test only happen on Sundays? ... April 24th 2016
I ALWAYS thought that Sunday was the holiest (metaphorically speaking) day of the week and that the 'la' was ALWAYS required. This is what my friends in Italy have corrected me on a hundred times.
Lets eat the cake now!