I'm so sorry for your loss Anna. Unfortunately I can relate all too well as I lost my grandmother in February of last year. Almost 2 years for me as well. They say the first year is always the hardest, and that was definitely the case with me. But just last week I was leaving my job at a local mall and I saw an elderly couple there that very closely resembled my grandparents, as vivid a reminder as there is, and it hit me like a ton of bricks and it was like I was back to square one in the grieving process.
Grief is like that. Eventually enough time passes where you feel like you've gotten through it, and then all of a sudden some little reminder pops up out of nowhere and you feel the pangs again. It's not always a straight line. Sometimes it's like 2 steps forward and 1 step back. This is totally normal, and it's okay to feel how you feel.
What I'm going to try the next time there's another reminder, is what someone suggested in a thread I just posted a few hours ago: accept that the sadness is there, but at the same time not dwell on it, try to remember a fun time I had with my grandmother, and move on to something else before my mood returns to sadness. I don't know if this is something you've tried before, but I think it's worth a try if you haven't yet.
And please don't underestimate the importance of you and your family leaning on each other during this difficult time. Or if you can't, at least have someone you can talk to about it when you need to.
This is my first real loss as well, so I can't say for sure if the loss itself gets easier with time. But I do feel that over time, the acceptance of the reality of the loss becomes a little easier to swallow, even if the loss itself doesn't, if that makes any sense.
Last year in our house, we had that same experience of there being an empty chair at the table at Christmas dinner. What we did was light our Christmas candle that we usually put in the center of the table every year, but last year we put that candle in her place at the table. It really made it feel like she was there with us in spirit, even if not in body. We also told funny stories about grandma that day. It might help to talk about and remember all the fun times you all had, maybe remember some of his jokes and laugh at them.
I hope this helps somewhat and I hope that you all can have a good holiday together.