Have you heard of it ?

Yes, I certainly have NanKate I have occasionally done it myself
I'm sure the family have activity days too.

Depends how often they do it! - sounds great to me as an occasional day!

My daughter does this occasionally with her youngest , they watch films together snuggled up with the duvet in pj's, share rare sweet treats and even order a takeaway my granddaughter loves it and looks forward to having her Mum to herself for the day. Other times they are out and about walking, cycling and doing all sorts I don't think there is any harm in an occasional 'duvet' day.

If they are usually racing round and being active it must be a treat to lounge about and relax especially if mum and dad are doing it as well and they are getting lots of attention.
Mum and dad need to unwind and relax as well if they have had a busy and perhaps stressful week.

I can`t see any harm in the odd PJ day , I couldn`t do it though I don`t feel right staying `undressed` all day .
Saying that , when my children were young they used to take turns having a day off school on their own just with me . We called it a special day . Having 4 I felt they got no individual time so it was a real treat for them to spend the day doing something nice without the other siblings .
It meant they got a day each every few months before anyone berates me . We did all sorts ie Museums , biking , cooking or what ever they wanted to do . They have very fond memories of special days .

I should have added that the GSs have Movie Night on a Friday and Saturday and can choose what they want to watch and eat popcorn at the same time, so it isn't all activity.

When I was about 6 I said to my Dad that I didn't want to get up that day and he said it was fine. About 5.00 pm I said, I will get dressed now, he said, No you won't it's not far off bed time! I never asked again.

Once in a while it's fine, surely?

Who are these parents who have children in the 3 - 7 age group who would want to spend all day watching films and relaxing? My DGC are on the go all day and rarely sit down to watch a film in one sitting, even ones they like will be watched in episodes.

If DS & DDiL tried to have a pyjamas and movie day, by lunch time DGS would be careering round the house like a banshee and DGD would be nagging to go out for a walk or do some cooking or similar activity.

Yes, DDIL has pyjamas days, I tried to have one, who cares as I live on my own, but was dressed by lunch time!!