I've had a few Christmas that were overwhelming for a few reasons, but I think I've got the hang of it now.
In terms of the prep, the key for me is just spreading it out over a couple of months. I enjoy doing it all, so I didn't want to reduce the volume of prep as some people do, just reduce the stress. Over a few months, it all gets done bit by bit, whenever I have time, and nothing feels rushed or pressured. Online shopping is a huge help, because it entirely removes the stress/pressure of feeling like I must find something in a few hours. Also enjoying the prep, not treating it like a job. Doing it in a way that my son can join in, not being a matyr and doing it all myself etc.
Then in terms of the festival itself, I've found that I just need to build in quiet time so that I don't find the whole thing too much.
If I'm hosting, that is easy.. I just do all the cooking. Lovely. I can escape at will from the crazy noise/bustle into the kitchen and quietly get on with my jobs, and then join back in as much or as little as I want.
If, like this year, I'm not hosting, then it is a case of just making sure that there are quieter places where I can enjoy smaller moments of peace. The Christmas day walk is lovely for this because not many go in my family, so it is nice and peaceful. And this year my son has some toys that are too small for other young children, so we've sorted him a spot in another room to play with them. Which sounds perfect, as I know there will be plenty of moments when I'll be able to get away from all the social stuff and just go and quietly play with my son, which will likely end up being some of the more special moments of the holiday.