To answer some of the questions, even the ones that are unnecessarily aggressive (“martyish wife” seems a bit harsh, I must say).
I will start by saying that one of the things I think happens with the mental load and other issues that land up with women being lumped with all the crap, is that we’re all programmed this way AND if often starts small and we don’t even notice it until one day we wake up and go, “hang on, how did we get here!?”
Over the years, as I've realised the patterns we were slipping into, I have had these conversations with DH. And he IS one of the good ones so has made efforts to change and take on more. But this one re communicating and explaining things to him is one I hadn't even noticed was happening or that it shouldn't be happening. Hence posting on here when I realised.
But for those asking if I’d said anything, yes, I did. Before I posted on MN! I pointed out exactly what was in my OP - he has done this before, he has had the comms, he has talked to DD. It’s not up to me to explain it to him.
In terms of what happens if if I didn’t do it: excellent question.And I suspect like with all issues to do with the mental load, the answer is that it would not be done. But the people who are affected are NOT the person not doing it - it’s the DC or me who are impacted. Which is why so often these things continue to land with women.
@headcheffer - not possible in this situation but yes, I have employed that tactic in the past to moderately good effect. Most notably when it comes to diary management!
For those thinking it’s a bit ridiculous for a minor school event - fair point usually. Except we had an odd logistics issue over here to do with his DS’ return from school and no keys and frail MIL wanting to attend … hence the need for a discussion. I didn’t mind the discussion. I minded that I had to explain how the event worked so we could then agree what would happen.
@coffeepott - I really really love “magic uterus”. Must work that in next time 
@version4needsabitofwork Actually, I am starting to realise that DH probably does have ADHD or some similar issue (executive function is definitely a thing for him) and we’re going through assessment process for DS. But it’s not a reason to be shite! 