Okay, when viola says,"What is a week-end?", I don't get it. What's so funny about it?

Well, some people who don't know the mostra so well don't understand that for the Crawley family work doesn't exist. giorno in, giorno out, is the same. Saturday is no different to Thursday, Monday is no different to Sunday. They don't work, none of them go to school, so every giorno for them is like a holiday for us normal people :D The middle class Crawleys who arrive at Downton in series one work, and Matthew has a job, so he works weekdays and has weekends off. So, pretty much, when Robert insists on him not having a job, he says he will have plenty of time on afternoons and weekends, and Violet, the dowager countess, granny, whatever te want to call her, doesn't know what a weekend is, and there is this look of surprise from Matthew as he realises that he is in a very different place now, among a very different class than he is used to. Hope this helped :) Thanks :D

Well it's just....she doesn't know what a weekend is because she never worked in her whole life! Everyday is the same, she can't imagine a "simple" life.