When is the best time of year to visit Croatia weather wise if you want to avoid the high season?
July and August are the hottest and busiest. September still has great weather, water is delightful, and most of tourists have left. End of May and early June are also great to travel - water is warm enough to go swimming and tourists are just starting to arrive.
A lot of people are answering to avoid high season. But, its double adged sword. They are mostly right. But even if high season is more expensive and crowded, there is almost no chance of rain to ruin your vocation (especially if you have very limited time to travel during the year), and all of extra activities are bookable (some operate only during high season) and the coast is swarming with concerts, traditional music and performances, fairs and fiestas (like the night of full moon in Zadar or fiestas for local patron saints on the islands). So it is up to you to choose what best suits you :)
Late September and October are a great time to visit, both from decent weather, and also all the harvests, both for wine and truffles, especially up in the region of Istria. It is getting a bit chilly in the evenings already, but still beautiful overall.
The best months for going around are May, June and then September and October.
If you like swimming in the warm sea water, it's September.
July and August are hot and very crowded.