One calendar several people can read
Why a schedule kept in one person's head eventually fails, and what a shared version needs to hold.
A calendar held in one head works beautifully until the head it is held in wants a day off. Then it turns out that nothing was ever written down, that the arrangement on Thursday exists only because somebody remembers agreeing to it, and that the person covering has to be briefed for forty minutes before they can be useful.
The problem is not memory. People who carry a household schedule in their heads are usually very good at it. The problem is that a schedule in one head cannot be shared, checked, corrected or inherited, and a caring arrangement will at some point need all four.
The calendar and the record are different documents
Most household calendars fail because they try to be two things at once. A calendar answers what is going to happen. A record answers what did happen. Mixing them produces a document that is cluttered with the past and unreliable about the future, so people stop trusting it, and once a calendar is not trusted it is not used.
Keep them apart. The calendar carries only forthcoming commitments. When something has happened, if it needs to be remembered at all, it moves to the record: a notebook, a running document, a page in the paperwork folder. Most things do not need to move anywhere. They just come off.
What actually belongs on it
A shared calendar should hold anything that another person would need to know in order to stand in for you. That is a narrower list than it first appears. It includes commitments outside the house, visits by anyone at all, and the recurring things that look invisible from outside: the weekly shop, the day the bins go out, the afternoon that is always quiet and should stay that way.
It does not need to hold the ordinary rhythm of a day. Nobody needs a calendar entry for lunch. If a substitute would need to know the rhythm, that belongs in a short written note kept beside the calendar, not in fifteen repeating entries that make the week look impassable.
Every entry needs an owner
The most useful column on a family calendar is not the time. It is the name. An entry that says Thursday, visit is a fact. An entry that says Thursday, visit, and the name of the person who will be there is an arrangement. Without the name, every entry quietly defaults to the same person, and the calendar becomes a record of one person's obligations rather than a plan the household shares.
Write the name even when it is obvious. Especially when it is obvious. The point of writing it is to make visible how many entries carry the same name, which is a conversation that is very difficult to start without a document in front of you.
Paper on the wall or something shared online
Either works. What matters is that everyone who needs it can see the current version without asking anyone. A calendar on a kitchen wall is excellent for people who come to the house and useless for a relative two hours away. A shared calendar on a phone is the reverse: easy to check from anywhere, easy to forget to look at.
If relatives are spread out, the practical answer is usually both, with one of them agreed as the real one. When two versions disagree, the household needs a rule about which wins that does not require a phone call to resolve.
The handover view
Once a month, look at the calendar the way somebody covering for you would look at it. Take the next two weeks and ask whether a relative who has not been closely involved could read those entries and know what to do. Where they could not, the gap is not in the calendar but in what has never been written down.
That exercise is uncomfortable, because it usually shows that a great deal of the arrangement lives nowhere except in one person. But that is exactly the thing worth finding out on an ordinary Tuesday rather than during the week you finally arranged to be away.