If your Dd really wants her friend to join in the Christmas fun then you could try this:
Go to the Christmas market, just you, Dh and Dd, pick out the tree etc.
When you return home, neighbour child arrives to help put up the tree and listen to Christmas music, have hot chocolate or a simple meal or whatever with your Dd and then you send her home. You then have the rest of the day/evening to lounge around in pj's, have more hot chocolate and watch movies, etc.
This way, you still have a special family day but it's extra special to Dd because someone her own age got to be a part of it for a little while. The majority of the day it will be just the 3 of you so you'll be keeping to your Christmas tradition, just changing it slightly for an hour or two to include Dd's friend.
There's nothing wrong either with saying 'No, come visit some other day instead', explain to your Dd that some occasions are family only, if you're sure that's how you want to keep things for the time being.
Keep in mind however, that your Dd will have her own memories when she's looking back at her childhood so weigh up the pros and cons of limiting these sort of occasions and traditions to family only as your Dd gets older. Going forward, since your Dd is an only child, I think you should consider including other dc in some of your family days out and family traditions, if that's what your Dd really wants.
Find a balance so it's sometimes just the 3 of you and sometimes other dc are included in these special and fun occasions.
(However, you do have to become better at setting out clear rules, it's not ok to have children ringing you or turning up on your doorstep at 7.30am or staying 'til all hours. Let your Dd have quality time with friends but don't let the children dictate when that happens)
I do understand about having those special Christmas traditions, it's similar in my home. It's just about recognising when to change things up a little bit so that everyone is happy.
Whatever you decide, I hope you all have a fun day.🎄