Every year in the last weekend of July, there is the Czech Ukulele festival:
http://ukulelefestival.cz/domains/ukulelefestival.cz/index.php/home
Which is always terrific!
The same organizer, Ben (who is from the UK originally) is also planning a festival in Italy in May.
This coming October, there is also the third Berlin festival;
http://berliner-ukulele-festival.de/en/
but I'm not sure they would still have room.
There is a festival in Caldogno, Italy, every 2 years; the next one in 2020.
There used to be a festival in Paris in spring, but I think that stopped last year.
In Belgium, throughout the year, there are several one-day events; I tend to hear about those too late.
You could try contacting Ukulelezaza (via his Etsy store might be the easiest way), he tends to be involved, or at least know about those.
Got a Ukulele has a big list every year, as well:
https://www.gotaukulele.com/p/ukulele-festival-calendar_11.html