I'm so sorry for your loss, I still remember that first Christmas afterwards.
I can only tell you my own experiences of my first Christmas after losing my parent, so if they don't feel right for you then that's fine. Take whichever little bits help, discard the rest. I do get what you mean about it all suddenly feeling pointless.
The first year, we repeated all our normal traditions and had Christmas in the same place with the same people, and in the same way as before. Albeit sadder because of their absence.
We talked about it and agreed that was what we wanted to do. For me, it was comforting because it was a way of still feeling connected to them that we were repeating things we had done with them. It made it a little bit easier to connect to happier memories of sharing Christmas with them rather than only the sadness of their loss.
It also, for me, was a symbolic way of showing the values they'd had hadn't been forgotten, if you know what I mean?
It was quieter, and slower, and yes, sadder, but I think part of healing. The build up was harder than the day itself. If you can, allow time for yourself in the days afterwards as you may be quite worn out by the emotion of it all.
In the years afterwards we gradually created different traditions and shifted away from replicating the past because that was too painful (longer term it felt like emphasising their absence) but in the beginning it was really important to me that I wasn't rushed away from having the continuity and keeping the sense of connection.
I don't know if I'm making sense. I know some people have completely changed everything for their first Christmas after losing a parent, but I couldn't have coped with that. It would have felt to me like I was "forgetting" them or trying to erase them, whereas I know other people find it easier to do things differently and remember them without the presence of all the old traditions making their loss feel even bigger.
Do you have a sense for what might be the least painful for you?
My main advice would be to just go with whatever your instincts suggest will make it most survivable for you, and not to beat yourself up too much. It's quite common for people to say things like "your dad would have wanted you to be happy" or whatever, but actually, I think all he would have wanted was for you to find a way to take care of yourself and cope with his loss in whatever way you need to do that. You don't have to pretend to be okay when you're not. You're human, not a robot.
That first year was hard, and I felt like my heart would never heal. I couldn't imagine how I'd ever be able to cope with having to do things differently in future years as our lives changed, but as time goes on it gets easier to carry their memory with you and it doesn't feel like you're leaving them behind in the same way as it does when the grief is raw. Things change, but you bring their memory and influence with you.
The way your dad shaped your life, the values you hold, and the person you are today will always be with you and part of you. In that way, you'll bring him with you whatever you decide to do.
Take care