This information is adapted from http://www.mit.gov.it/mit/site.php?p=scioperi, listing strikes of national scope. I'll list important local strikes if I become aware of them. Strikes that have been announced, then postponed or cancelled are also noted. Strikes from past dates are listed in the second entry, including cancelled strikes, which are crossed out, to give an idea of how often postponements and cancellations happen.
Often these strikes are only called by some of the several unions involved with a transportation agency, and operations do not shut down completely.
This page reflects my interpretation of the information given as of the date last updated, and I assume no responsibility for any variations on the scene on the date of travel.
For train strikes: Here is Trenitalia's general statement in English about how essential train services operate during a strike. This PDF lists long-haul trains that are promised to run during all strikes. Two days before a strike, the Trenitalia site generally posts a list, prominently from its home page www.trenitalia.com , of trains confirmed or cancelled for that date. They also list trains for the different regions of regional trains that will run, generally one or two trains in each direction in the main commuting times.
June 4, 2012
National transportation strike (rail, public transport, sea) 4 hours to be announced.
June 6, 2012
Meridiana flight attendants 10 a.m. to 2 p.m.
June 22, 2012
Milan airport staff, 24 hours.
July 5, 2012
Milan airport staff, 24 hours
Meridiana Fly flight attendants, 24 hours
July 8-13, 2012
Cargo transportation strike (possible effect on gas stations), from 10 p.m. July 7 to July 13.
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