May Day! May Day!
The public transportation service will not be normal on the first of May, a public holiday. Instead, it will run in Milan from 7 A.M. to 7:30 P.M. (too bad that the ATM English language pages of the web site are not complete...you would have missed this news, had you depended on THEM!).
Beware, too, on the 30th of April...Saturday hours will be in force on this pre-holiday day.
Check the websites for the public transportation of other cities.
Hope this helps!