I think parents do over estimate how much attention non-parents pay to half terms etc.
I would respectfully disagree. There are very few - if any - people who are oblivious to the metronome of the school year.
Obviously as a child you are aware of it. And even as you leave full time education (which could be early 20s) there is every chance you - or friends - will have younger siblings still doing the time.
Then you move into the world of work where it takes about 10 seconds to remember how the school year runs as colleagues fight it out for annual leave slots. That's even if you are unobservant enough to wonder why your commute can treble in term time.
From there you - or family or friends - will become parents yourselves. That's going to be at least 14 years when you are directly exposed to the school calendar.
After that when the kids piss off, there are still those colleagues fighting over annual leave.
And when you leave work, there's the grandparent phase. And - guess what - that too will march to the beat of the term time tempo.
Even having my morning tea, I am aware of the parents walking their kids to school, and there absence during half term.
(really just proving a post I made yesterday that the fossilised remains of the western educational calendar was centred around getting the harvest in).