@Sirzy
They are adults. You need to make new traditions and realise things have changed
This. Sorry
@Time4change2018 and you have every right to feel blue about this, and it is sad when things change, but as kids become adults, they will form new traditions. I found that, and it made me feel a bit down for the first couple of Christmases, when DD decided to spend Christmas day AND Boxing day with her boyfriend - and not me and DH.
But you get used to it.
We see extended family around 18th-20th December and swap presents, and then see DD and her boyfriend now, on 22nd/23rd December, and have a pub lunch with them. Then Christmas Eve, Christmas day, and Boxing day it's just me and DH. We have got used to it now, and enjoy the 'alone time.'
Also, you will (possibly) have new Christmas memories in the future when grandkids visit. 
@BaronessEllarawrosaurus
It's hard when your children grow up and are no longer there for these key events but it has to happen, been there already and yes I do miss my daughter when she isn't with me at Christmas but I'm also incredibly proud of the independent young lady she has turned into. It's a reason to celebrate, as well as shed a few tears.
Beautifully put! ^ 
@penguinwithasuitcase
Nothing more horrible than dreading Christmas because you have to decide where to go and you know the other parent will be upset.
100% this ... ^ That's why I never EVER let my DD know that I was a bit upset that she didn't want to see us Christmas day, the first time she did it. As I said, I don't mind now, and neither does DH. it was just a bit of a shock to the system when we didn't see her on Christmas day (for the first time.) We had been with her on Christmas day for 24 years! As many posters have said though, you have to let go eventually................