Several come to mind...
The wine festival along the Mosel in Bernkastel-Kues is big, fun, and full of locals. Planned that one into our schedule.
The Cannstatter Volksfest in Stuttgart is what Oktoberfest used to be, it is more personal, fun, but still a big fest. That one was planned as part of an itinerary that included Oktoberfest and brewery visits.
On Mykonos the local historical society puts on a Harvest festival in September, it has been 10 years or so since we were there, but it was a wonderful day of music food and wine. Surprisingly at that time it was not very big, more like a celebration thrown by a big Greek family. That one was pure chance, but probably our favorite.
Another "event" to look for, and is a bit of chance, is traveling in Europe during either World Cup (FIFA) or Europe Cup. If you are fortunate enough to be in a country that is in the Quarter/Semi or Finals, then impromptu festivals spring up when the game is on. A number of memories from English Pubs, to small Italian towns, to big Plazas/Piazzas with jumbotrons.