Let's be honest. That weekend, no matter how much you prepare will be nerve wracking and nail biting. Not to say you can't prepare. That is so important to do. But it will be new and stressful. The ways you can prepare is to talk every day. Ask every question. Be very openly and completely honest.
In my opinion I think you need to let him give you as much as detail as possible. Obviously not go into move by move sex positions. But let him know what he needs to tell you how he felt. Maybe, if you feel up to it, the intensity level. Maybe have him chart these answers out in a 1 to 10 level concept. 1 being not intense and boring and 10 being so amazingly out of this world. Use that for any worries and questions you might have. Talk to the girl, not just him. I know that is hard but if you are able to I definitely recommend getting both points of view of how they both feel towards each other and towards you. You need her perspective and an open line of communication with her. Who knows, you might even become friends which would be incredible. J and I are.
You should know I am a mono. I am not poly. My husband is poly and just recently broke up with his poly gf due to circumstances beyond their control. My story is much more complicate than this so I won't get into it. If you want to read more you can under Life Stories and Blogs. I am the one called "Blog of the MOno Wife"
But when DH and J were together I made them both talk. I still make them both talk. It has always been important to me because in the beginning when I learned about them I didn't ask all the questions. I asked alot. But not enough and I only talk to him. And because of this my mind managed to create scenarios that weren't real. But they were nightmares about what the two of them would do and how they felt and that I might be being pushed out and there was no place for me and I wasn't good enough. These things would torture me over and over again in my head until I would explode on almost an every other day basis. At least weekly if nothing else.
Then I started talking. I started taking advice and ideas from these boards. And I started listening. And I started talking to J. I talked to both of them every time they were together. I made him talk as soon as he got home before he lost any thought or nerve of telling me anything I should know. So I would sleep better. I made her talk to me the next day so that everything was fresh. And it helped. My nightmares started going away. I slept better at night. J and I got closer. DH and I were communicating better. It was so hard hearing some of the things they did together. But I am glad I knew instead of let my imagination take over and create twisted tortorous tales. My nightmares were daymares and everything in between. But after awhile I grew better.
Next weekend get busy. Go shopping, go to a spa, call up some friends. Whatever you do don't just sit in front of the tv unless it is a movie a thon that has you so enthralled you can't do anything but pay attention to it. Start planning now. It is so important to not let yourself linger and wait for him to come home. He will come home to you.
And when he does let him come home to you with open arms. It will make it easier to talk. Hold each other while he tells you. And even if you feel the urge to shrink into a ball away from him don't. Feel each other's closeness and love.
As I said he and J have broken up. It was a painful process but they have learned to be friends. They still talk on the phone. They still love and care about each other. But I still talk to him about it.
I know that he is poly even if he can't have J. So if he ever finds someone else, I pray I can accept her somehow and that he and I are still as open as we have learned to be here.
Good Luck and PM me if you ever need to talk.