For starters let's get the terminology right. Don't be running around asking the good people of Barcelona about Carnival, it's Carnestoltes. As a side note, Catalan is widely spoken in Barcelona so no one really expects you to use anything but your own language.
Something very special that happens every year without fail and without any doom nor gloom, just plenty of elegance, colour and tradition. It is the event of the year in Venice's calendar and it explains why you'll find rows and rows of masks in the souvenir shops and how you will find the magic of Venice is still alive.
Maybe the masks of Halloween aren't so very different to the masks we all wear, every day, at every time of year. I do this all the time. We all do, I think.