Ceitdh
Rather then giving an opinion, as you've been given enough of those already, I am going to make a suggestion.
You said that your elder daughter hasn't told you yet, that it was your other daughter that has mentioned this. That suggests to me that your elder daughter might be agonising over how to tell you, worried about how you will react.
My suggestion is that instead of waiting for her to raise the subject, you approach her instead, saying that now she is a mum herself, you realize that things are different, and it's time to change things. You could tentatively suggest doing something the day before instead (which I think is what you have been told she would like to do anyway?)
Show what a good mum you are by not making an issue of something that really isn't a big deal in the scheme of things.
I think it's regional differences...

