June or September?

Water should be warmer in September.

I would choose early to mid September. As you get closer to October things will start to shut down and it will also cool down. We went mid September and it was warm during the day and cool at night. My husband was comfortable swimming but it was a little cool for me, so that may just depend on what you are used to.

We would have liked to go in June because of better waterfalls at Plitvice. However, just found incredible airfare for next September, so that is when we are going.

you're finding flights from the US for next September already?

Yes. Got a great flight Cincinnati to Paris nonstop and then on to Dubrovnik on Croatia Air. Return is Zagreb to Paris and then nonstop to Cincinnati. Ticket was $828 booked through Air France. I have been looking at prices for a long time, and the typical fare has been $1400+. Check google flights.

That is a good price. We are in Akron and usually drive to Toronto in order to get better flight prices and nonstop flights to Europe. So Cincinnati is not out of the question. Thanks!

WE spent the better month of September in Croatia, and it was warm. I wore shorts my whole time there; the last few days it was cool enough for jeans. Croatia gets pretty warm/hot from June onwards...

Spent the end of September in Montenegro, perfect weather.

South will be warm in early September, more moderate in late September. Istrian coast can be very cool and windy in the evening by late September. I've been to Plitvice 3 times in September, and it's always been beautiful and not overly crowded. Haven't been to Croatia in June, so can't compare it to September.

Thanks everyone. We ended up getting a fantastic price on flights, so September it is!