Been there many times to visit family, but never this early. Usually late summer. But end of May it would be nice and warm already. Probably mostly upper 70s on the coast.
But it is very well priced there for food and lodging. Rentals are pretty expensive though and air fare directly into Croatia's airports tend to be more expensive which is why I like to land in nearby countries like Austria or Hungary and rent a car there. Saves me easily several hundred bucks.
Where do you plan to go in Croatia?
I always love the old coastal cities such as Dubrovnik, Korcula, Trogir, Zadar, Sibenik for sight seeing but my favorite is Split because its a great blend of old and new. Great night life there. But for best night life is probably Makarska