E.R.
My son is at his dad's roughly every other weekend. He used to also be there on Wednesday night- but just when it would be a routine, something would come up and gradually they would just keep canceling, etc. ( I think they don't like having to get up extra early to get him to school the next morning- it throws off their 'routine') .
Everytime anyone talks to my ex and his wife, they go on and on about how hard it is that they don't get more time with his son and how much they wish they could see him more- but they never really TRY to spend much more time with him or make a lot of extra effort about it. It seems like when it is easy for them, they do it, but we are constantly having to readjust things around their schedule.
My son also goes with his dad for a week in the summer. Again, I would be happy to have him there longer, but they have never asked to take him longer.
It's good to have everything in writing- but we are also flexible on the little things, like Memorial Day, etc. I usually end up with my son for the random days off school that seem to come up all the time. We alternate major holidays like Christmas and Thanksgiving every other year, which has always worked out well.
Just remember that the goal is for the visitation to be stable and predictable for your kids. If he can't be absolutely sure he can commit to taking them on a certain day, he should not get the kids hopes up! A good mediator should understand that- you want to be fair, but the kids are the most important thing here!! Good luck!