People are great, aren't they? Especially when they decide to not show up at the same place you are, at the exact same time (it's also cool if they're not wearing the exact same thing we happened to pick out that morning)!
Most places in the world would love to increase their tourism numbers. Countries, regions, and cities spend millions and millions of dollars on marketing trying to lure you into visiting their beloved corner of the earth. Spots on popular tourism website's 'best places' to visit lists are coveted year in and year out. However, there are a few places on earth that would prefer that you just stayed home, or visited somewhere else (or at least stopped by during the off season!).
Venice and Barcelona, it seems, are just too popular! I would call the 'off season' for both of these European cool kids mid October to late April, or early May.
There can be a lot of benefits to visiting in the off season too... cheaper prices (getting there and once you arrive as well), less crowds and shorter lines, happier locals, and you usually will have more opportunities to experience local cultural performances and festivities.