To plan your trip to Venice, you do not really need to look at the Venice climate data. You can visit Venice any time in the year.
If you want our advice, do not go in summer – it is burning hot in the sun and crowded by people. The best time is in early spring or late autum as it is sunny, warm and less crowded. Walking around is much more fun then.
Please look at the graphic with the long term average climate data for Venice for more information about the temperatures by month.