Pitchwife, you're right in terms of your reference. In Volume I,
Chronology, of their
The J.R.R.Tolkien Companion and Guide (2006), Christina Scull and Wayne Hammond deal in detail with Tolkien's 1955 trip to Italy with his daughter Priscilla. There is a lot of detail from Tolkien himself; because he kept a journal of the trip, which he called
Giornale d'Italia.
On 27th July 1955, he wrote to Rayner Unwin, '
I leave the North Kingdom [England] for a rapid excursion to Gondor [Italy] on Saturday morning, but shall, I hope, be back by August 16'. (
Chronology, p. 462)
His and Priscilla's trip to Italy is covered on pages 462-74 of the
Chronology, and pages 463-8 deal with their time in Venice. On page 466, we read that after both had dinner with Christopher and Faith Tolkien, who were also staying in Venice, they returned to their hotel. Tolkien's account speaks of it being
a beautiful balmy night...Venice seemed incredible, elvishly lovely - to me like a dream of Old Gondor, or Pelagrir of the Númenorean ships, before the return of the Shadow.