Croatia has a continental climate with cold winters and hot summers, especially inland. Near the coast it is milder. So it rather depends on the kind of holiday you want. For example, there are ski resorts and there can be good skiing especially in January, mainly near the capital city, Zagreb.
For summer holidays, July and August are the hottest months, but they are also the most popular with tourists so the coastal areas in particular are very crowded. If you like a bit more peace, a very nice time to visit is spring, especially May, when it is usually quite warm - typically over 20 celsius - and the spring flowers are out. June is also a good month, and in June there are a lot of interesting events such as dance festivals.
Equally, September is warm and pleasant but not too busy. There is also a growing number of tourists who like to visit before Christmas to enjoy the atmosphere.
You can find some useful information about Croatia here.