There are people in the world who operate from a position of dissatisfaction with everything. The fact that she's dissatisfied with holiday time dinner scheduling isn't the issue. She's dissatisfied with everything. You only get to see her dissatisfaction at the holiday dinners. If you lived with her you'd see she's actually unhappy with everything and everybody.

I used to shop at a particular grocery store for about five years. In the last year of shopping at that grocery store I felt annoyed that I always seemed to pick a broken grocery cart. It either didn't steer right or it was hard to push, as one of the wheels was always damaged. And I got a broken grocery cart about 85% of the time.

Today I realize that 85% of the shopping carts at that store are broken. They didn't do maintenance on them. So I didn't have bad luck. They had bad shopping carts.

I think your step mother has anxiety and depression. And it manifests as dissatisfaction with everything, which you think is her being dissatisfied with the dinner scheduling. Even if you were to do everything her way, she would still find something to be dissatisfied with...food, arrival time of guests, parking, tablecloths, etc.