You don't have to win this one
Memories, money, the children, in-laws, bedtime and when it's more than arguing
The rule holds everywhere. A few kinds of argument need their own paragraph.
Memories. Chapter 9 is the argument. There's no record, so the first question can't be answered, so drop it. If the memory matters because of what it shows, talk about what it shows, as a thing about you and not a claim about Boxing Day.
Money. Money disagreements pass both questions more often than most, and they're the ones that deserve the slot in chapter 22. What they don't deserve is to be fought as facts. "You said we could afford it" is a memory dispute wearing a money argument's clothes. "Can we look at the account together on Saturday?" is a money conversation.
The children. Two things. Arguments about facts in front of them teach them what chapter 18 says. And disagreements about the children themselves, school, bedtimes, screens, are the real kind, and they need the slot, away from the children, and a decision you can both hold to, because a child who sees two parents disagree in front of them learns which one to ask.
In-laws. What his mother said and what her father meant are the most contested facts in most households and the least checkable. They also aren't about the in-laws. They're about whether your partner is on your side, and that's a question you can ask directly, on a good day, without Boxing Day in it.
Bedtime. The argument at ten at night, when both of you are done, is the worst version of every argument in this book, and a later book in the series is about the belief that it has to be settled before sleep. Here I'd say only that "let's leave it till the morning" is a drop, and that the morning nearly always finds the fact smaller.
The partner who argues to be right. If you're reading this because you live with the calendar rather than hold it, the exit is available from your chair too. "You might be right" said by the person who was going to be shown the calendar anyway ends the spiral just as well, and it's not surrender. It's declining to be in the argument. What you can also do is say, on a good day, outside any argument, "when you show me the calendar it makes me feel small", which is a real disagreement about a real thing, and deserves the slot.
When it's more than arguing. If the arguments in your house end with one person frightened, or if being right comes with contempt, name-calling, threats, or control over what you can do or spend, this isn't a habit and this book isn't the answer. The National Domestic Abuse Helpline is on 0808 2000 247, free, at any hour. And if the arguing is ordinary but has become most of what you do together, couples counselling exists, Relate is the best-known provider in the UK, and going isn't an admission that anything's ending. It's two people getting a third seat that stays.