This Sunday is the annual 'National Day' of ADSI where 400 mansions, castles, gardens and similar historic 'dimora' all around Italy are open to the public. Most are free and some of the smaller sites need reservations, and a few have special restrictions. This event takes place every May and most of the sites will repeat next year if you are trip planning. Look at the sites by region here:
https://www.associazionedimorestoricheitaliane.it/eventi-dimore/347698/xiii-giornata-nazionale-domenica-21-maggio-2023-clicca-qui/?lan=en
Thanks for posting.