Do it when it works for you. I had mine in Vegas a couple of weeks ago even though (at the time of planning it) we weren't getting married till November. I had a couple of teachers in my group so June worked best for everyone.
I wanted mine a month before so that during the month before my wedding I was just free to do all my last wedding things but that didn't really happen so we did ours a couple weeks or so before. But I'd think for a cruise maybe a month before.