Zagreb to Plitvice Lakes by bys

Looks like that's a holiday called Corpus Christi.

If you take a bus to Plitvice in June, unless you take a super early bus, be prepared for a Disneyland-type of experience with lines and crowds. I wouldn't do it myself, unless I could stay overnight at the park itself and enter at 7am when the park first opens.

Renting a car is probably not expensive (even for a day), and the drive from Zagreb to Plitvice is easy, partly on a fast modern toll highway.

That is very strange. I tried the websites of BusCroatia and GetByBus and found either nothing or just the one bus. There are many buses on the 19th and 21st.

June 20th is a holiday--Corpus Christi. I wouldn't have expected that to mean no bus service to Plitvice, but perhaps that is the situation.

I don't know whether you are thinking of a day-trip or were planning to stop for a night at/near Plitvice on the way to somewhere like Split. Day-trips to Plitvice are very much not recommended because of the unbelievably crowded conditions during day-tripping hours--which I imagine would be even worse on a holiday, because a lot of the visitors to the park seem to be Croatians. Plitvice is gorgeous, but if you are marching in lockstep with thousands of other people on the boardwalks, it is not a very special experience. I would skip it if I could not spend the night and see the park first thing in the morning (or partly very late the day I arrived and partly the next morning). I say this after having taken a day-trip to the park from Zagreb in 2015; I am sure it has only become more crowded since then. And I wasn't there on a holiday.

Thankyou for your replies. will take that advice on board.

You can easily get there by taking early bus to Karlovac and another to Plitviče.