I was told that some hotels may not turn on their air conditioning until a certain date. Is this true? Have you been able to get AC when you needed it?

It is true that hotels probably won't have their AC on in May. However, depending on where you are going you may not need (or miss) AC. I usually travel in May and do not stay in hotels with AC and have never needed it.

Many hotels turn their AC on at a certain date - or a certain temp - usually not controlled in each room as in the US. We were once in a hotel where they turned the AC off at night - since the staff went home. Had to create a huge scene the next day (as in threatening to move to another hotel) to convince them they HAD to leave it on at night - when the guests needed it to sleep - rather than during the day when we were out touring. (It wasn't just us - there were at least 10 guests complaining - so they finally agreed to leave it on at night.)

If heat is going to bother you contact the hotel and find out their details.

We also suffered in September when they had turned A/C off for the winter. One of the requirements I had for choosing a hotel was A/C. They had set dates for turning A/C on and off regardless of the weather.

Why would you want air con in May? Italy is not tropical, May is still spring time.

"Weather" is whether it will or not be hot in May or even Sept anywhere. Seasons are screwed up everywhere. What's usual can change from year-to-year. Best to check with your hotel if/what schedule they may have. Even if they have room controlled a/c. Some newer hotels do!