I went to all of them because I have young kids and they love that stuff. It's a fun day out. Also gives us an excuse to wear all of the arms and armor we've collected, lol.
There are lots of these in Austria, too, BTW. There was a two-week-long joust fair/festival by Millstätter See in early August!
In truth, these festivals are all over Europe. They're just not well advertised, so you have to look for them.
The next one in the region, by the way, is the medieval-themed Advent festival at the castle in Friesach. (Austria.) I'll be there for sure.