Portions of public beaches in Italy are given to beach establishments under a concession agreement. These establishments are called “Stabilimenti balneari” in technical terms, but are known in everyday language as BAGNO (plural BAGNI) or LIDO (plural Lidi). So one such establishment might be called “BAGNO ROBERTO”, or BAGNO BLU, or LIDO ANYNAME. Each regional government regulates concessions differently. All require that some portions of the beaches to be left free of concessions. For example a region might say that every 200 mts given in concession there must be 100mt left free.
The first 5 mts from the water is state public property that must be left free and unencubered. You can walk and sit there but nobody can plant an umbrella with chairs on the first 5 meters from the water edge..
If you go to the satellite image on Google Maps you can see the portions of the beach given to a concession because it will have a few rows of nicely lined umbrella. The portions where you just see scattered people but not nicely lined umbrellas, are the free portions.
I doubt that in May many concession establishments are up and running. They usually start operating in June, but if the weather is good and the place is busy, maybe some will start working in late May. If they are, tha is where you can rent your chairs, your umbrella, and even a changing cabin. Italians rent a changing cabin because they rent by the week or by the month and use the changing cabins as a storage for beach stuff (when I was a kid I stored so many toys inside our cabin, my mother couldn’t even get in to change her clothes). If you just go there for a day and have no kids’ toys to store, don’t waste money renting a cabin. You can change your costume in the bathroom or on the beach using a towel around you