Spring in Croatia is sublime and April, May and June are absolutely lovely. Autumn is probably best time of year as the landscape is made up of a gold and red pallet so September and October are unmissable. July and August can be hot so you will have to start early to miss the heat of the day, although the towns and villages are busier with many seasonal festivals on which is great. During July and August you can expect much more traffic on the road and crowds in the towns and on the beach.