I was thinking about the future for my DCs. I hope they and any partners would know the date that their children leave on their last day of primary (even if they couldn't be around due to work).
Still not getting why. I've had 3 leave Primary (and 3 leave secondary, come to that). As long as one parent knows when the CMer needs to stop - or, in many cases, when the holiday care needs to start - ,then I still don't see why the other does, particularly. 
There is no way, in most families where parents work, that parents can be at every event the school puts on (plus Cubs, or football matches or swimming galas or dance shows etc etc etc). It is very normal to 'tag team'. If dh is taking one of mine to camp then I don't hold in my mind the details of that specific day, as dh 'has it'. If I am taking another to a football match, then dh doesn't need to know the details, because I am covering it.
Same with school. I didn't take them on their first days of Primary, because I was working ironically as a teacher so had no freedom to 'go in late' or 'book the day off. It wasn't an issue as my dc have 2 parents and dh, at the time, could arrange to go in late. By the time they left Primary, I was working PT and was able to go to Leavers assembly etc, but dh wasn't. No issue - someone was there for them. I have no recollection of how they made their way out on the last day though, as it just wasn't "a thing" for any of them, or for us.
I agree with theStuffedPenguin that it does seem really bizarre that him not seeing her on one evening is a big issue for you, when you are then taking her away for 30 days without seemingly expecting any trauma. 