I am having difficulty determining if there are daily ferries between Venice and Rovinj in May of next year. We are planning on going on a Friday and returning on a Monday so if there are only ferries on two days per week we are in trouble.
Well, you might be in trouble. First, I have not used this ferry. But I did travel a bit around Croatia last May, and what I found was that there was a dramatic difference in the frequency of ferry sailings based on the divide between "high season" and "low season". High season was (I think) June 1 - August 31. Anything outside of that window was generally considered "low season". In low season, some popular ferry lines run infrequently, and some don't run at all. We found this was also true for bus schedules. I have no idea about the route you mention, but I would not be surprised if it was not running at all in May, never mind every day. If your trip schedule does not have plenty of flexibility built in, you should find out for sure and adjust plans accordingly. Unfortunately, even a few months ahead, that can be challenging. Next year's schedules may not be available yet, and even if they are, it's a surprisingly complex system with many different companies, and it's difficult to track down solid info. Try here: Croatia Ferries - I found this a very useful source for ferry info. Good luck.