What a shared calendar does better than any paper system
Annual checkups slip in multi-person households because the knowledge lives in one head: the person who took the cat last year remembers, roughly, while everyone else assumes it is handled. A shared family phone calendar fixes the visibility problem completely. Every adult sees the same due month, the same alerts fire on several phones, and a conversation in a shop or car can be settled on the spot. The calendar's job in this system is purely timing and coordination. It is a poor archive, so it works best when you are deliberate about what goes in the event and what lives elsewhere.
Build one rolling event per cat
Create a separate all-day event for each cat, titled with her name and the words annual checkup, placed in the month she falls due. Attach two alerts: the first well ahead, around six weeks before, labelled as the moment to phone and book, because routine slots at busy practices go early; the second a few days before the appointment itself once it exists. In the notes field put the practice name and number, the items the vet asked to recheck, and any handling note, such as this cat needing the first appointment of the day. One event per cat matters: a combined pets event blurs which animal is actually due.
Assign a booker, or sharing becomes diffusion
The known failure of shared systems is everyone seeing the alert and each assuming another person acted. Decide, per cat or for all cats, whose job booking is, and write that name in the event itself. That person confirms the booking back into the event by replacing the placeholder date with the real appointment time, so the calendar always shows whether a due month is merely approaching or actually arranged. If the named booker is away when the first alert fires, the visibility of the shared calendar is what saves you, because someone else can see the task is still unbooked and step in deliberately rather than by accident.
Roll the event forward at the appointment, never delete it
When the checkup is done, resist deleting the event in the tidy satisfaction of completion. Instead, edit that same event: move it to the same month next year, refresh the notes with this visit's weight, vaccines given, and recheck items, and reset the two alerts. Rolling the original event forward preserves a visible chain and prevents the most common calendar error, a reminder that existed for years quietly vanishing because someone cleared old entries. A short dated line added to the notes at each roll builds, over several years, a surprisingly useful summary of the cat's annual pattern.
The calendar holds dates; the record needs a home
A phone calendar will faithfully tell you a checkup is due, but at the appointment it cannot produce last year's blood results, the vaccination card, or the weight history the vet wants to compare. Keep those documents attached to the cat rather than buried in a camera roll or email thread, in a form any family member can open when they are the one holding the carrier. That document home is the role KinsLe Pet Passport fills for each cat, keeping her records together in one packet that sits alongside the calendar system and travels to the surgery with whoever takes her.