We are also leaving next September 4, arriving in Venice 4 days early, taking the train to Ravenna on Saturday morning and staying overnight and taking a taxi to Porto Corsini, the cruise port. Just watch the trains, some require 2 transfers some only one. There is no direct train to Ravenna from Venice, however the fare is very inexpensive.
There are trains leaving every hour on Saturday.
You can also get your covid test at the train station in Venice before you board to Ravenna. We decided to arrive the day before the cruise so we could tour the mosaics in Ravenna, much of the town will be closed on Sunday so there wont be much to tour or shop. It also makes for a relaxing morning before cruise check in.
