Vaporetto

yes they do, but if the Grand Canal is blocked the service reduces and those lines don't run.

Which holiday?

24/7 and 365 days a year.
Most Venetians do not own a boat and since water cabs are very expensive the vaporettos are the only way to get around.

During the night and on holidays, they run (sail?) on a reduced service, like conventional public transit services all over Italy.

Unlike their wheeled counterparts, the holiday schedules of vaporettos are not that reduced. Nevertheless if you are going to visit Venice on Xmas day or on May 1st, you can wait up to 40 minutes for a boat.

http://actv.avmspa.it/en/content/orari-servizio-di-navigazione-0

Thank you, these replies are helpful :)

There is a handy app, CheBateo? (Which boat?) that links to real time vaporetto timetables. Plug in your departure and destination vap stops, and the app tells you which boats to take and when. Handy if you are travelling to the outer islands, say from Lido to Torcello.

There are times during the year when special events impact the availability of the vaporetto. Once, in late May, we arrived at P. Roma from the airport and planned to take one to the hotel. There was a big regatta of some type and nothing else was allowed on the Grand Canal. Walked to hotel and then enjoyed watching the races. Might be wise to check and see if a special event will affect your plans.