The established tourist season in Croatia is from April to September; winters are very cold and unpopular for travelers. September may be the best month to visit, as the crowds and heat are over with but the sea is still warm. September visits also let you take advantage of off-season rates.

Zagreb is likely to be nice during September and the cultural season is in full swing at this time. May and June are also ideal times for all outdoor activities (except skiing). Watch out for battalions of school students on class field trips at the end of May and beginning of June.
July and August are the most expensive months to visit Croatia as the tourist season swings into gear.

If a summer trip fits better into your schedule, you will be able to take advantage of extra ferries as well as the season’s organized sight-seeing and adventure excursions. July is also the most popular month for festivals in Croatia.