We still have no official word from the school this morning Bathsheba
My DS's teaching staff were "not sure" this morning when I asked them. They have another event going on today though, which required a particular item of clothing.
I found that out on Friday at almost 5pm, when a text came through saying children who owned this item of clothing could wear it today to take part in a special event.
We were halfway down the motorway, for a weekend away, in a place with no shops selling this sort of clothing and no wi-fi to order it.
So that was annoying. 5pm on a Friday night, giving notice for something quite particular and needed for 8:45am on a Monday morning. 
Children who don't one this item can't take part. The mood in the playground this morning was not good. More than half of the school feeling excluded from something today and nobody having a clue about Thursday.
On the way home I spoke to a parent who had heard from another parent who heard from someone else that a teaching assistant in Year 3 had said that on Thursday the children can wear any character costume but also have to take in the book their character comes from.
I've started putting a costume together now, out of stuff we already own, in the hope that this is right.