We have just returned from Ventura. You do need to book for freedom dining, but can only book one night in advance. We ended up sharing most nights as there aren't many small tables, but it was nice to meet different people every night. If you turn up "on spec" you can end up waiting for up to an hour. You can just turn up any time in the Beach Hut (self-service). Our 8 year old preferred the children's tea time in the Beach Hut which runs from 5 pm and then he could go to the kids club while we had our evening meal. Enjoy - we did.
You turn up for dinner when you want, and sit by yourselves or share as you want. You are not forced to eat at a time determined by the cruise line, and you are not forced to sit with people you do not know or have nothing in common with. (Can you tell I am a huge fan of Freedom Dining?
You can also book a table for, I think, two nights at a time. If you want to dine at a certain time you may wish to do this. If you don't you may find that at certain times you may have to wait. You will be given a pager in this case so you can go and have a drink at the bar. I hope this is correct. I am not a fan of Freedom Dining but this is information I have picked up from the P&O site.